Motionsports

Skiing takes some time to get used to. It won't feel right at first and it probably won't feel right when you're done.
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My method to simplify this was to use a Kinect friendly chair, which I will refer to as the chair method. The chair method is used Course 2,5,6,7

If you've ever played The Gunstringer while sitting down and the controls worked fine then, it'll work here as well. Basically its a chair that lets you sit up right and still reads your motions. (can't slouch back)

What this does for you here is that when you sit down on your chair, the Kinect will think you're staying fully crouched. This will save wear and tear on your knees as well as automatically earn you tiny gradual idle boost gauge and boost for you as well when applicable.
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A typical race will start with you pushing off 3 times followed by sitting down and gently steering.

Course 2 is the hardest ski course in the what am I supposed to do? category. Course 5 is the hardest ski course in the technical skill category.

Bounty Super G (Course 1)
Final Score 409pts
Gold Tokens 24/30 240pts
Silver Tokens 43/76 129pts
Bronze Tokens 4099 40pts

Course Score 332 pts is a Silver.

Course Tips: You have to turn wildly to kill your speed so that you can inch your way into the medals. Gold is worth the most by far but don't underestimate the value of a straight line of silver or bronzes.

Ski Cross (Course 2)
Final Time 01:13:15

Course Time 01:14:31 is a Platinum

Course Tips: Take everything you know about skiing in this game and forget it...

You want to start with a semi-slow start so all 3 of them are in front of you at the start but not to the point where you aren't within inches of catching them. You don't want them to ram you from behind in the start of the race.

Afterwards, you want to cut every corner possible. All staying on path does is give you more boost for each flag you pass. However, boosting = maybe 1-2 second better time? You get more for cutting corners and you'll eventually get boosts using the chair method. Its not to say to miss every flag on purpose but it is definitely not a priority.

Bounty Ski Cross (Course 3)
Final Score 282pts
Gold Tokens 14/16 140 pts
Silver Tokens 34/57 102 pts
Bronze Tokens 40/95 40 pts

Course Score 268 pts is a Platinum.

Course Tips: A tougher, stricter version of Course 1. The key is to learn the placement of the gold medals and how to get them while still being able to get most of the silver and bronze.

Ski Jump (Course 4)
Final Score 262pts

Quick and painless. Its practically a free Diamond. Well, you'll get the diamond if you try for the Go Ahead and Jump achievement. Basically you want it so that your boost ends just before you jump.

Super G (Course 5)
Final Time 01:05:51

Course Time 01:06:34 is a Platinum

Course Tips: Please refer to the chair method above. The less you move, the better (while still taking the turns) as turning slows you down.

You can go off course and cut corners as long as you make it back in before the flags. There is only one real spot you can do that in this course, the gentle long left turn.

Super G Extreme (Course 6)
Final Time 01:21:35

Course Tips: Chair Method. More of the same as Course 5, only easier.

Ski Cross Extreme (Course 7)
Final Time: 01:12:41

Course Tip: Play Course 2, if you Diamond that one, this will be a cakewalk. Just cut corners, avoid collisions and whatnot and you'll be fine.

Whenever you go under 35-40 mph, you can push off to speed up a tiny bit more. It doesn't always work but it does help when it does work.

Some general tips:

1) If you can utilize MemoriesofFinal's "chair" method effectively, then do it. It will be much easier on your knees. But if you're like me, and couldn't, then you want to focus on keeping your knees slightly bent and hands parallel to each other, resting on your hips.
2) Utilizing boost is key, there are only 2 situations when you want to start it up; (a) when you're coming on a short corner and your speed is low. This will allow you to make a sharp turn while still maintaining a descent speed and upon exit, you will still have a much better speed then it would have been if you didn't use the boost; and (b) on long straight shots where you can clear a good distance and maintain a fast speed even after you exit the boost.
3) For levels with snow blowers that mess up your vision and you're somewhat blinded by snow; you simply want to cross your hands across your face, this should immediately wipe off your mask and you're good to go.
4) For the bounty levels, it's important to hit as many golds as possible (10 pts!); I'd suggest building your run around hitting 90% of them and then tweaking your run to hit as many silvers/bronze along the way.
5) Move as little as possible! You want to have slow movements whenever possible to maintain high speeds. If you can master turning with as little movement as possible, you'll do a lot better for yourself.
6) Keep yourself planted! do not move your feet, stay planted in one place.
7) Hit as many flag gates as possible. crossing through a gate will increase your boost gauge, if you can manage to hit the majority of them and combine it with utilizing your boosts as mentioned above. You'll be better off.
8) Do knee stretches prior to your sessions. Skiing is the most physically demanding sport in this game, but it will take a toll on your knees if you are not used to being in a bent position.
9) Keep water on hand, you want to stay hydrated.
10) Since most of the challenges will not allow you to utilize the "4 player" method which helps keep you in a rhythm between loads; you can still minimize the loading time between sessions by restarting after you finished the race (before entering you course stats). You'll want to do this after you see the "Saving" notification in the top left corner. Simply press the xbox guide button on your controller, exit back to motionsports and it should have the pause menu up for you to restart, cutting some of the load time out while still keeping your previous score.

Courses:

Bounty Super G:
Reference my tips above regarding bounty and this should not be that difficult. I'd recommend killing your speed by having your arms stretched out. If you do not have a strong upper body build to keep your arms extended for long periods of time, I'd suggest keeping them in motion as long as they remain above your collar bone.

Ski Cross:
There are some short corners worth cutting. But for the main part, hit your gates, time your boosts, and avoid hitting the other skiers (unless in boost since hitting them won't affect your speed if you're "boosting"). Avoid hiting walls, flags, and skiers and you should find your ninche it get the diamond.

Bounty Ski Cross:
Reference my tips above, you definitely want to hit at least 13 gold tokens with the rest being focused on silvers. Be sure to wipe off your mask and the key area once you mastered hitting the gold tokens is about mid-way where you need to be going somewhat fast to hit about 10 silver tokens (30 pts), it's a curved right turn where you can't go slow in order to get the silver tokens. Master the gold token route you do and master hitting these silvers and it should make things easier. If you're not hearing the commentator saying you're "blowing the course away" or something similar (except for the first time before the first snow blower), you'll want to restart.

Ski Jump:
I have a guide for the 784 ft jump which I suggest you refer too. But in short, boost between the 2nd and 3rd Motionsports banner, and jump before the first red line (you'll see on your screen either a "too late/nice/perfect" when you do it) and mimic the pose on the screen as best as you can.

Super G:
Oddly, this took longer than the Extreme version for me. Just be sure not to move as much as possible and utilize your boosts as mentioned above Thankfully, the cuts are so difficult but there may be some where you'll want to activate your boost to avoid the sudden drop in speed.

Super G Extreme:
There is one sharp left turn you need to worry about in the course. It comes about midway through and you'll want to stay on the inside and make a sharp turn, this will reduce your speed, but you'll still hit the gate and be moving whereas if you stayed on the outside, you're more than likely going to have to make such an extreme cut, that your speed will drop below 10 mph. Be sure to wipe your mask off when snow gets in your face!

Ski Cross Extreme:
This is not that difficult if you've gotten diamond in everything else. Just remember to utilize all my tips and guideline above. There is a sharp right turn about 3/4 into the track, I suggest utilizing one of your boosts at that point, specifically when you're about to cut so that when you do activate boost, not only do you keep you speed up, you'll clear the next section easily since it's a straightaway.

Happy Hunting :)