If you shove to the players 4-bet, he will likely call you preflop. Whereas if you decide to just flat call, and then shove on the flop you give yourself that extra chance to win the pot. In this hand the player had QQ preflop. If he flat calls and the flop comes A K 2, then its a lot easier for him to fold his hand. But if the flop comes the way it did he can shove on the flop, this way if he put the guy on a lower pp or AK they may fold here. The point im trying to make is that if your going to shove preflop anyway, whats the difference between shoving on the flop?