For this trick simply divert the course of a ball so that it goes under an arm & pops straight up on the opposite side of the pattern.
Stand without juggling. Hold both forearms out in front of you as you do when juggling a Cascade.
Take one ball, keep the empty hand perfectly still & swing the ball low & wide underneath the other arm. Your throwing arm should end the scoop when the halfway point of the forearm just touches the halfway point of the arm you are throwing under. Throw the ball so that it rises up in a straight line to the same height as a normal cascade throw (eye level) & it should fall to the point where you'd catch in a cascade.
Practise the move in a cascade. It may help to make one throw a little higher than usual before you throw one under the arm to make up for the increased dwell time (dwell time is how long you hold a ball for before you throw it).
As should be the norm by now do one at a time, same ball each time (this is Juggler's Tennis but with the tennis ball going under the net), every left & every right & then every single throw. When you can do every throw you can almost immediately do Chops & it is very useful for learning the Spiral & Mills Mess.