I first saw this trick enthusiastically performed by Sean Gandini at our Christmas party show in 2000 (not quite as enthusiastic as Chops though). Sean learnt it from the trick's creator Jay Gilligan. The reverse of this trick, Luke's Lobotomy, is also worth learning. This pattern is a shape distortion of 504.
Stand with your arms crossed & with two balls in the bottom hand & one in the top. Place the wrist of the hand on top in the crook of the elbow of the lower arm.
In one move throw one of the balls in the lower hand using the all of the arm. Make a High Throw that peaks above your head & crosses over to the other side of the pattern. At the same time open the fingers of the hand on top & pull it back over your other arm's elbow allowing the ball to roll off the end of your fingers so that it will fall on the outside of the lower elbow. The upward movement caused by the hand on the bottom will cause the ball in the top hand to rise up a little way before it falls. If you purposefully try to throw the ball from the hand on top you will more than likely throw it too high &/or out to the side. If you want to run this pattern for a long time you have to throw the under arm throws high & the upper arm throws really low.
Uncross your arms then recross them the other way round, catch the ball that you rolled off the end of your fingers with the same hand that dropped it as an Under the arm catch. Place the wrist of the arm on the top in the crook of the elbow of the arm on the bottom. Finish the step by catching the high throw in the hand on the bottom of the stack as well & then stop. The ball sitting in the upper hand after the two throws doesn't move in this step. 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 underarm throws high & the overarm throws low.
Now do the same again, cross your arms, one ball on top two on the bottom, top wrist to bottom elbow. Make the high throw Under the arm & roll the single ball off the end of your fingers, uncross & recross your arms then catch the rolled ball. Then immediately throw the ball in the hand on the bottom Under the arm as a high Over the top throw on the outside of the ball in the air. At the same time roll the ball off of the hand on the top just like before. Catch the first high throw in the lower hand as you start to uncross your hands, recross your hands again catching the low throw in the new lower hand. Now just keep going.
For bonus points try juggling Jay's Jumble on one side & Luke's Lobotomy on the other (Crossed arm 504). Then do two rounds of either pattern to switch which way round your arms are crossed over.