Juggle a Cascade & pick a ball. As that ball falls into one hand instead of throwing it underneath the next ball throw it straight up on the outside as if you were juggling a Two in one hand columns pattern so that it will fall down on the outside of the cascade. Throw the next ball with a cascade throw a little higher than normal. Swing your hand underneath the other arm to make the catch. Make the next cascade throw as normal with the hand that you just made the catch under & uncross your hands to resume the cascade.
At first this trick will seem very awkward & you will probably bang your arms together a lot trying to fit the throws into place. Move the arm that you catch under inwards over the top of your catching hand so that the scoop as you make the throw doesn't get in the way of the catch. After you have caught the underarm ball it may help to continue the outward scoop as you uncross your arms & throw it as an Over the top throw.
Learn this trick with both hands. This move is essential to learning Mills Mess & many other patterns where the arms cross & uncross.
Instead of uncrossing your arms after an under arm catch, throw the ball straight back up as an Under the arm throw. As you make the catch lower your hand & lift the ball straight back up again so that it rises to eye level. Uncross your arms & resume juggling.
This trick is used in the Reverse Spiral, Mills Mess & many others.