Luke Burrage has nicked someone else's trick (Jay's Jumble), played it backwards & named it after himself. Out of pity I'll humour him. Underneath all the arm crossing the basic pattern is a simple (4x2)(24x) (not 504). If you have already learnt 531 Mills Mess you will have a hard habit to break. Just tell yourself: THE UNDERARM BALL IS NOT THE HIGH THROW!!
Stand with your arms crossed & with two balls in the top hand & one in the bottom. Place the elbow of the hand on top in the groove made by the wrist of the hand on the bottom.
In one move throw one of the balls in the top hand using the whole arm. Extend & retract at the elbow to make a High Throw that peaks above your head & crosses over to the opposite side of the pattern. In the same movement keep the lower hand in place by the other arm's elbow & throw the ball straight up so that it falls back down on the same side of the pattern. The position of your arms should limit the height of the underarm throw to a few inches. To run this pattern for any length of time you need to keep the over arm throws high & the under arm throws low.
Uncross your arms then recross them the other way round, catch the Under the arm throw with the same hand that threw it, which after recrossing your arms should now be on the top of the pile. Place the wrist of the arm on the bottom against the elbow of the arm on top. Also catch the high throw in the top hand as well & then stop. The ball sitting in the lower hand doesn't move at all (yet). You should now be in the mirror of the position which you started in. So do the same again & practise the two throws starting from alternate sides, keep the overarm throws high & the underarm throws low.
Now do exactly the same again, cross your arms, two balls on top one on the bottom, top elbow to bottom wrist. Make the high & low throws, uncross & recross your arms then catch the low throw. Then immediately throw the ball in the hand on top as a high Over the top throw on the outside of the ball in the air. In the same movement throw the ball held in the hand on the bottom as a low throw under the arm just like before. Catch the first high throw in the upper hand as you start to uncross your hands, recross your hands again catching the low throw in the new upper hand. & repeat.
You'll notice that each ball is alternately thrown low under the arm & then high over the arm. Also notice that the high throws on their own make a high Reverse Cascade pattern.