Combo basketball dribbling moves need to be sharp and quick to work. Basketball drills like this one are great for working on that. The reason why is because you force your hands to move quicker than they normally would have to. With the tennis ball you have to make the dribble move and catch the tennis ball before it hits the ground, this trains your hands to be quicker.
The toss double behind the back tennis ball drill is going to work on doing a double behind the back move, and also work on your hand coordination. If you are able to do this move while focusing on the tennis ball, then in a game the move should be no problem.