In this trick the throws are exactly the same as in The Spiral & the arms cross in exactly the same way as they do in Mills Mess all you need to do is put the two tricks together.
Start by juggling a Spiral pattern & just concentrate on the Under arm throw, you catch the ball on one side of your pattern, then you carry the ball to the other side in a nice smooth scoop & then throw it back to where it was caught. Now instead of making an under the arm throw, catch the ball, carry it OVER the opposite arm to the other side & then toss it back to the same side. Carry on with the Spiral. The arm motion is a little bit awkward, instead of a scoop you have to carry the ball in a straight slice across the front of the body & then throw the ball using the wrist only. This move should feel familiar to you if you can do the Reverse Spiral.
Keep juggling a Spiral & occasionally pop in an over arm carry in place of an under arm one. Gradually increase the frequency until you are alternately throwing under & over the opposite arm.