The kicker creates topspin when he strikes the ball for an onside kick. By kicking the upper half of the ball and driving it downward, he makes the ball roll forward through a couple of bounces before bouncing high into the air, giving his team a chance to recover it.
The kicking team may not touch the ball until it has traveled 10 yards down the field; if the kickoff takes place at the 30-yard line, for example, the kicking team has to wait until it crosses the 40-yard line to touch it. If they touch it earlier, it is automatically down and the receiving team takes possession at that point. The exception happens when the receiving team enters that 10-yard area and touches the ball first; if that happens, the ball becomes live, and anyone may recover it.
The strategy with the onside kick is to try to get the ball to bounce high in the air so that it comes down somewhere along the end of that 10-yard zone while giving kicking team members time to sprint there after the kick and attempt to get the ball. While the receiving team is usually able to recover the ball on an onside kick, this strategy sometimes works when the receiving team is not prepared for the onside kick and has already turned to run back and block for a returner, or when the receiving team's players are unable to control the ball after the high bounce. Then, the kicking team's players get there and recover the ball.