I really like this move because even though it looks cool and interesting in a show, it's actually really easy to do. The move can be done with any square number of marchers (4, 9, 16, 25 etc.) but it looks best with a large number of marchers.
To do this move first form your corps into a perfect square. Then select each individual diagonal line and hit the "plus" key the number of times equal to the number of marchers you have selected. In this example the middle diagonal is 8 marchers, so I selected them and hit "plus" 8 times, then 7 for each line of 7 marchers and so on. Make sure you do all diagonals one at a time. Trying to save time by expanding both of the 7 lines, then both of the 6 lines, etc. will cause your form to come out looking like a blob and doesn't really look that good. I hope this helps!