Once you are able to completely backtrack, you'll return all the way back to the first area where you were being tested. The ball that you had to put on the center switch is still there. Warp into it and press your back and forth to make it explode.
Alternate Easy Method: You can actually gain this as your first achievement. Once you gain the ability to warp, go back to the ball and destroy it. You do not have to figure out this power later with the corpse.
Tyger7 Credits: Nutmeg for figuring this achievement out