from the FAQ:
Q: How do the double team options work?
A:
Double teaming a player will put more defensive pressure on the player selected, reducing his shot attempts/efficiency and possibly forcing more turnovers.
On the other hand, if a player is double teamed, that means somebody is open - giving the open player(s) more open shots than they'd normally see.
Setting a player to 'always' will attempt to double team him when he has the ball. Setting a player to 'leading scorer' will only attempt to double team him if he is the leading point scorer for his team.
Keep in mind that the defense won't be able to successfully execute the double-team on every possession.
Q: What happens if I set everyone to double team always?
A: On any given possession, you may be double teaming up to 2 players. If you've selected to always double team 3 or more players and they are on the floor at the same time, nobody will be double teamed .
Q: What if I select 2 players to double team always and 1 to double if he's the leading scorer - which takes precedence?
A: When you choose to always double team a player, that takes precedence over any other setting. The only exception is if you have settings which would cause 3 or players to be double teamed at the same time. For example, if you have 2 players set to double-team always and 1 player set to double-team when leading scorer (and he is the leading scorer), when those 3 are on the floor at the same time, no one will be double teamed .