The toss double crossover tennis ball basketball dribbling drill is an advanced tennis ball drill, but once you have mastered some of the easier tennis ball dribbling drills it is a great drill to move on to. If you aren't challenging yourself in your workouts, then you are holding yourself back.
In a basketball game when you attack your defender off of the dribble, you have different dribble moves, and you have counters to those dribble moves. The crossover is a basic move and the double crossover is a counter move. If the defender slides over to cut you off on the crossover, then that is when you can cross the basketball back over, making the move a double crossover.