This one is very similar to the Mills Mess with reverse cascade insert, but in this trick you put in just one throw of a Shower inside Mills Mess to totally transform the pattern.
Start by juggling Mills Mess, make the throws to just above head height to give yourself time to watch what is going on in the pattern. Watch the ball that is thrown Under the arm. Now ignore it & concentrate on the other two balls (it was the easiest way to get your attention in the right place with words :). Notice how the two balls follow each other's paths from one side of the pattern to the other exactly as they do in the Shower.
If you have read my Mills Mess tutorial you will probably remember me referring to the three types of throw that make up Mills Mess. One is the back toss, which is the one made by the arm on top when your arms are crossed over that goes back to the other side of the body. Another is the Reverse Cascade throw, which is the only throw made when the arms are uncrossed. For the next bit I'd like to call the back toss SHOWER THROW 1 & the reverse cascade throw SHOWER THROW 2.
Keep juggling Mills Mess & watch the ball being thrown under the arm (I mean it this time), catch it but don't recross your arms. Instead, throw the ball horizontally into the opposite hand at the same time as the other hand throws the ball it is holding. This bit is exactly the same as a simultaneous shower, keep your hands apart & make the horizontal throw with the wrist only. Then catch SHOWER THROW 1 with your empty hand, then with the other hand make a reverse cascade throw which follows SHOWER BALL 2 while crossing your arms. Catch SHOWER THROW 2 with the hand on top & make an underarm throw with the hand on the bottom & resume Mills Mess.
That is one insert on one side of the pattern. Practise this move on both sides. Catch & pass a left hand under arm throw & resume Mills Mess then catch & pass a right hand under arm throw & resume Mills Mess. Gradually build up the frequency until every under arm throw is caught & passed.
For something really cool, Penguin Catch SHOWER THROW 2. The penguin catch is made with the arms crossed by the upper arm, you'll need to hunch that arm's shoulder forward & maybe twist your body to the side a little too in order to comfortably make the catch.