As you can probably guess from the unwieldy title, this trick involves Mills Mess & Reverse Cascade.
Begin juggling a reverse cascade. Without breaking the cascade throw one ball under the arm. Catch the next ball before uncrossing your arms as an under the arm catch then uncross your arms, make sure the next throw is thrown over the top. This under arm exchange is exactly the same as in Half Mess (some people may have spotted this by the fact that I have copied & pasted the instructions from that tutorial). Do this on one side & then the other until you can do it on both sides easily.
Continue juggling a reverse cascade & start counting from 1 to 9 as you make each throw. You will probably find this next step easier if you increase the height of your throws to just above head height. Make every ninth throw the under arm throw of an under arm exchange just as you have been practising. After the first time you throw under the arm the '1' becomes a Back Toss (see the Mills Mess tutorial if you are usnure what this is).
Gradually decrease the length of the sequence start from 1 to 9, then try 1 to 7. The last throw is always the under arm toss. For the full Mills Mess with a reverse cascade insert the count is 1 to 5.