It's basically a play where you are essentially pot committed, and out of position-- so instead of putting all the money in preflop, you open shove and give your opponent an opportunity to make an additional mistake by folding.
Your stack is 1000, with blinds at 25/50, you open raise for 150 with TT (850 left in stack). Your (loose) opponent re-raises you to 450 (let's say he has a big stack, of 6k+). Instead of shoving all-in here you simply call the additional 300, then shove all-in for your last 550 (into the 900 pot) no matter what the flop is-- it's possible that your opponent may fold a missed big ace, or even a hand like JJ on an board such AK2.
Instead of getting to see five cards, where a hand like KQ is 50/50 vs JJ, he only gets to see three (IF he folds). It's just one extra way to win, and often doesn't work, as your opponent may recognize that you are shoving with any hand, but still-- it gives them the opportunity to make a mistake.
edit: one thing that I should mention: if you are going to chicken out and not shove on the scary board, then you are better off putting it in pre-flop you YOU get to see five cards: don't make the play on yourself!