A flop sequence would be a select series of skills performed on one pommel. They will always count in element group IV.
Direct Stockli, Direct Stockli, Pommel loop. --------- "D" flop
Direct Stockli, Direct Stockli. Pommel loop, Pommel loop. ------- "E" flop
Direct Stockli, Pommel loop, Direct Stockli, Pommel loop. ------ "E" flop
If you go to the website www.NGJA.org they have a fantastic video on explaining flops.
You can also use russians in flops. There are a few different variations, but you can only do them and get credit if done and put in to the routine properly. There are rule I believe stating no more than 2 can be done in a routine, and they cannot be connected. You must break them up I think (don't know off the top of my head right now) by stepping down off the pommel, and performing another skill. The skill may even have to be a "B" or higher.
Check out that web site though. Very helpful. It will be located on the left pages scroll box entitled Current Flop Video. It is a few Minutes long.