Trials HD

Here a video. Not mine, by Youtube user Mr360Games
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Alright... since neither of these two incredibly helpful* lads thought to actually tell people what to do (and, frankly, I don't like video solutions), here we go:

The Medium level "Workshop of Secrets" has you enter an elevator. You can get off in one place rather near the ground, one higher which may or may not be a shortcut... or stay on the elevator a bit longer yet. The third floor is the way to go, literally. Following that path, eventually you get to a ramp leading directly to a fence and a large Y in the background. Bail (with, you guessed it, Y), fly over the fence and try to hit the third box from the left. If you do it right, the box will fall through the floor and the achievement will unlock.

You don't exactly need to land in the box to get the achievment here.
I landed half in and half out of the box and it started to slide down the slope without me. As long as the box slides down the slope and explodes in the rubbish bin achievment is yours. :-)

All of the above solutions are great.

Just a little tip to add is that you can still control your rider using the left stick by pushing up or down. Takes a bit of messing around with it but I was able to move from the 1st box after a poor attempt all the way to the 3rd. When I got the rider hitting the 3rd box on the side facing the camera it triggered the box falling through the floor and using the left stick the rider down the hole aswell. Achievement unlocked.