The idea here is the same as with any popcorn pattern: juggling 4 clubs for a very short period of time while your partner is juggling 3, then passing them the extra club.
In this pattern, it's always the same club that is passed between the jugglers as a single. When the pass comes in, flash your 3 clubs (as triples in theory, however it's easier with floaty doubles), catch the pass, feed it to the other hand, pass it back to your partner then catch everything that comes down.
7 clubs, popcorn 5551 : <5 5 5 1 3p 3 3
3 | 3p 3 3 3 5 5 5 1>
Tip: make your flash on doubles (make sure you don't throw them forward) and concentrate on throwing the second double (left hand) high enough to calm things down. Also, be careful with your single passes; there's a natural tendency to throw them a bit too high.