Epic Mickey 2: The Power of Two

A Dahl Engineering Corridor (D.E.C.) is a portal from one area to another, first introduced to you in Mean Street by Gremlin Jamface. They are distinct by being like playing a short platform mini game through a giant toy box and there are 5 in total but only a couple are story related. Here they are in order:

#1 - Story related. Early in the game you will meet Gremlin Jamface in Mean Street North. Give him some Scrap Metal to fix a windmill by the Cinema, enter the repaired door and the D.E.C. entrance is on the floor, it will exit in OsTown.

#2 - Story Related. Once you drain the thinner from the fountain in town, use the lift to get to the floor beneath. Have Oswald reprogram the panel on the door and the D.E.C entrance is in the floor, it will exit to the Rainbow Cavern.

#3 - If you destroy the substation in Rainbow Cavern, your only way of proceeding is through the D.E.C shown in the cutscene. If you repaired it, just proceed through the projector instead but when you emerge in Angel Falls, use Oswald to glide down off the platform and down to the left where the indelible ink well is. You can then go through it in reverse and it will count as complete, just use it again to get back to Angel Falls.

#4 - Back in Angel Falls again, there is another entrance to a D.E.C. on the opposite side of the area to the exit of #3. Go past the lava maze until you get to a platform with a Blotworx enemy on it, dispose of him then use the crankshaft to open the mouth of the Devil statue. Hop up inside of it to find this D.E.C. which will take you to Disney Gulch.

#5 - Depending on if you have been nice to all Spatter enemies and not upset Ghost Ian at any time, you will actually not be able to access the entrance to this one, a bit like #3 but you once again find the exit instead and go through it backwards. To do this, wait until you get to the area in The Floatyard with the pirate ship and the bongo's on the wall. To get to the D.E.C. exit, drop down over the side of the cliff to reach a ledge and hop down to the next one and you'll see the exit from the overhang.

Notes:

1: You can't just enter one and back out, you have to go in one side and exit out the other for it to count.

2: Purely from a fluidity perspective, I would use projectors to get all the Film Reels on your big playthrough and get this one instead on your Off Track playthrough. Do it this way will save you jumping back and forth between lands so you don't miss anything but ultimately the choice is yours.