A strip in football is the action of ripping the ball away from the ball carrier, forcing a fumble. Once the ball falls to the ground, it is considered a 'live' ball and can be recovered by the defense, causing the offense to lose possession. This is known as a turnover.
Once the ball pops loose and falls to the ground, it becomes a race for the ball between the offense and defense. If the offense recovers, they keep possession of the ball, albeit at the expense of having start the next play from behind the line of scrimmage where the ball was recovered. If the defense recovers, the play is officially ruled a turnover and possession is granted to the team that has just forced the turnover.
Gaining possession of the ball can be very tricky, as there are often multiple players vying for the ball and causing it to roll all over the field. The best way to secure the ball and gain possession is to fall on top of it and protect it with the body by wrapping around the ball in a fetal position. This ensures that the ball will not come loose when a pile up ensues, in which players from the offense jump on top of the defender that has just recovered the ball in order to steal it away before the referee can sort out the pile.
While an attempt to strip the ball can yield great results, it is also a risky maneuver that can lead to the ball carrier gaining extra yards. This is because stripping techniques often prevent the defender from wrapping his arms around the ball carrier and making a secure tackle. Thus, if the ball carrier has a tight grip on the ball and is able to maintain possession through contact, it becomes easy to break the tackle and continue running up the field.