You use it when you're short stacked (4 BB is a good example) and feel that a push wont force out your opponent. So you call preflop and push on the flop regardless of what you have, hoping that your opponent completely missed the flop and doesn't want to give you any more chips (3BB worth). It also helps to act first on the flop so he can't agress, and to also have a decent hand preflop

(doesn't mean you shouldn't push on the flop if you miss).
This works because if you go allin preflop it's pretty much a guaranteed gamble, whereas if you stop-n-go there's a chance he'll fold. Still, don't expect a fold on the flop too often.