This is the easiest trick to learn and looks quite impressive for such an easy trick.
Step 1 : Juggle normally.
Step 2 : Once you have caught a ball, move your hand across, under your other arm and throw.
Step 3 : Move your hand back to catch the ball that is coming down (this is the bit that seems fast when you first start)
Where the ball that went under your arm goes is not that
important as long as it goes somewhere you can catch it.
Different variations on this trick usually depend on where
you throw the ball eg. over to your right hand or straight
up in the air.
The hardest variation of this trick is to do it continually
or alternate between your left and right hands.