As the outer ball swings
up over your thumb for a top hit,
loosly catch it with your middle finger.
As you catch the outer ball, straighten your index finger
and let the inner ball slide out of your palm between your index and middle fingers.
This is where the balls switch places.
The "new" outer ball swings around your thumb, back between your index and middle fingers,
and up over your thumb a second time for a top hit, where the cycle begins again.