with the guy on the perimeter, and prevent the kick out (or steal the attempted desperate kick out). But if you only "weakly" double team, a player like Williams uses one of the double-teaming players to essentially box out or eliminate the other one, and then goes through both for the layup.

The purpose of the double team, is to do it hard enough that the offensive player is not able to move, and must pass it out. If he can move, then he splits the double (like Jimmer used to do a lot), or goes right or left, and basically eliminates two defenders (out of five). If he splits them, they are now handcuffed in the spot, and are now the ones not able to move with the play, like twin statues with their arms up. If they play a "weak double team," and the offensive player goes right or left with athleticism, then one statue runs in to the other one, and is eliminated from the play, and if that defender hits the other one hard enough trying to pursue a guy like Williams, he also eliminates the guy he hits. Now, the offensive player is free to drive to the open hoop (the other 3 defenders no where near), while the other 4 offensive players crash the boards (knowing what's next), or he can kick it out to a wide open 3 (and TTU had good ones).

Double teaming is like blitzing in Football - you better get home, or be absolutely precise on rotating to the likely recipient of the kick out release valve, or you will PAY THE PRICE!
But "lazy" double teams or lazy blitzes will get you burned.