Here is an exercise to practice juggling balls.
Be sure to keep your upper arm still. Use only your forearm to create movement.
Now let's go a bit faster. (Throw, throw. Catch, catch.) Now let's do the same with the left hand. Do the same with your left hand
Now we're going to integrate the individual rotation movements to make a complete pass. First, we'll rotate the ball left. Then, right. Then we'll change from right to left.
Right, left. Left, right.
The idea is to toss the second ball (once)/before? the first ball has finished it's (ascension)/descent? into the other hand.