So when you block up the Jinxes like this:
sss
sws
sss
Where the S = Solid and W=Walk (where the Jinx is placed), the Jinx will be invisible. Yeah, I can't explain why, but they are invisible. In this puzzle, once you press a switch the solid blocks lower, the Jinx is free. However, once the Jinx is able to move, he is visible... So it is just when he is trapped.