When this dirty trick is used, the ball is rigged to explode about five seconds after the play begins. The last three seconds are signaled by a countdown beep. While this gives the defense a warning to steer clear of the ball, the offense may also use this as a hint of what's coming. As long as the offense called a pass play and the ball is still behind the line of scrimmage, it's possible to negate the trick by throwing the ball out of bounds or even turn it against them by throwing the ball towards them a la Bombs Away. As soon as the ball explodes, the play is over. If the ball was in play at the time, the ball will be spotted at the point of the explosion. If the play results in a touchdown, the score still counts and play continues normally. If the ball went out of bounds or explodes in the middle of a pass, it's considered an incomplete pass and play continues on that assumption.
This dirty trick is especially effective against a team focused on running like the Orcs of Hazzard. There are few countermeasures to a runner holding a Hot Potato, and while the player can score holding a Hot Potato, the ensuing end zone celebration is almost certain to kill the player: a handy tactic against a team low on players. It also works on punts; when the ball detonates, the team on defense will start their possession from the point the opponent punted the ball.
In the classic children's game of "Hot Potato", the player holding the "hot potato" when time runs out is eliminated. The term has since been extended to encompass any object that is not safe to keep in one's possession for any length of time (such as, in this case, a ball about to explode).