In the 2-2-1 full court press, 4 players usually begin in the backcourt of the offensive team with only the center on the defensive half of the floor. The two guards are usually closest to the other team's end and the forwards closer to half-court forming a sort of box in its initial setup before any rotations.
The front line players at the top of the press have a main role of forcing offensive players out of the middle of the floor and to the sidelines where traps should be created with the help of the middle line forwards. The other role of the middle line players if a trap does not occur is to create tough passing lanes to advance the ball forward in. The center in the back needs to stay put to keep the offense from an easy layup or also intercept a slow pass if possible.
Any press including the 2-2-1 is issued to create turnovers and more importantly disrupt the other team's offensive tempo. More time is used to get the ball into the offensive half of the court when a press is used. The defensive team wants to force as many turnovers as possible however they cannot over pursue these opportunities as it leaves them open for easy baskets if they are not successful. The offensive team will also tire more quickly when facing a press.
The 2-2-1 press does have some holes however. If overextended or played incorrectly it can allow very easy buckets in transition. It can also affect a team that does not have a deep rotation with fatigue and extra fouls that wouldn't have been picked up in the half court. Overall, the 2-2-1 press can disrupt the opponent and force turnovers while also conceding some easy buckets if not executed properly.