In American football, referees can signal when the defense either makes an illegal play, warranting a penalty, or scores. Only a few signals are specific to the defense in a football game, but the referees know these signals well.
Illegal contact.
The referee extends his arm and an open hand forward to signal that illegal contact was made.
Illegal crackback block.
The referee strikes with an open right hand around the middle of his right thigh, preceded by a personal foul signal, to signal an illegal crackback block.
Illegal cut block.
From the side, the referee uses both hands to strike the front of his thighs to signal that a player made an illegal cut block. When he uses one hand to strike the front of his thigh, preceded by a personal foul signal, he means that an illegal block below the waist occurred. When he uses both hands to strike the sides of his thighs, preceded by a personal foul signal, he means that an illegal chop block occurred. Finally, when he uses one hand to strike the back of his calf, preceded by a personal foul signal, he means that an illegal clipping penalty occurred.
Interference.
The referee, with open hands vertical to the ground, extends his arms forward from his shoulders to signify pass interference or interference of a fair catch of a punted ball.
Safety.
The referee puts his palms together above his head to show that the defensive team scored a safety. Look for this signal whenever the offense is operating near its own goal line with the quarterback and running backs actually lined up in the end zone.
Uncatchable pass.
The referee holds the palm of his right hand parallel to the ground and moves it back and forth above his head to signal that a forward pass was uncatchable and that no penalty should be called.