A player may not break contact with an opposing player once contact has been initiated as long as the player is still directly in front of him. Doing so will result in a penalty, with the exception of a defender chucking an opposing receiver within 5 yards of the line of scrimmage, which is legal.
Chucking is very similar to a stiff arm, the main difference being that it is a penalty in most cases. Chucking is done with one or both arms, while a stiff arm is typically done with one arm. A player will use the chucking technique in order to break contact with an oncoming defender, which is why it is illegal. The act of chucking is performed by first extending an arm (or both) into the defender, and then immediately flexing, or retracting, either one or both arms. The quick shift in the pressure put forth against the defender causes said defender to lose momentum, and possibly even their overall balance, thereby breaking the original contact.