The trick is in the pressure plate you are standing on. You cant move off it without triggering the explosives. Or, well, that specific character cannot move.
But you dont have a single character, do you?
That switch underneath the box is the only switch.
btw, that trap can be solved with single character... but the games achievements dont admit it but falsely claim i needed my companions! Not true! i did it all myself!!!
:P
