[bones]

The theory behind it is this- Say you have a decent hand out of the blinds and someone in lp makes a raise. Your stack is such that you are likely to have no FE preflop. Say you have 1800 and he opens to 900 at 150/3. He's pretty much never folding if you shove. But he might fold if you call and shove any flop.

The reason why it's done with pairs is that you're trying to fold out hands that will beat you on the turn and river. Here's an example.

Button opens for 900 with KJ. You have 1700 in bb with 7s. You're likely ahead of his range and with the blinds and stacks the way they are, you pretty much have to play the hand. So you call. Let's say the flop is 952. Or A84 or 345. Or pretty much anything not containing a K or J. It's gonna be very tough for him to call your shove. By doing this, you prevent him from seeing his 6 outs twice on the turn and river. You can do this with any hand really, but it's most effective with smaller pairs IMO, because they are the hands that least want to see all 5 cards.

If your hand is strong enough where you don't mind seeing all 5 cards, then obviously get the money in preflop. This is just another line to consider with marginal hands that come up at certain stack sizes.