This happens when he throws the ball away to avoid getting sacked (tackled to the ground with the ball still). If he tries to throw the ball away, he can throw the ball towards:
- the down marker on the sideline (the bright orange pole that marks the line of scrimmage and the first-down line)
- a receiver
If he throws the ball and it isn't close to a receiver (I think a receiver has to be within 2-3 yards), then he gets a penalty for Intentional Grounding.
The signal for Intentional Grounding (when the ref announces it), he will:
- put his hands about 8 inches apart - facing each other
- move his hands from up by one shoulder diagonally down in front of his body to end by the other hip.