I confess I don't remember whether it's canon or fanon that Imperius works by eroding your will, by making you feel like you don't care, that resistance isn't worth the trouble, rather than by the electric-shock-dog-collar principle that seems to be the case with the House Elves' enchantment.

It's canon. Before he learns to throw off the curse, Harry describes exactly what you say, feeling as though nothing matters, he doesn't care what happens. He takes "verbal" (though mental) commands from Crouch, but actually acts under his own steam; he isn't forced bodily.