Since clicking is hard yet you still hunger for an answer...

Flex system is basically a solution to guilds/FCs that have too many players for 1 static group but not enough to fill a 2nd group. So instead of a team of 8 and another team of 8, you'd have a team of 8 and 3 people sitting around hoping someone didn't log on that night. The raid itself (including gear) would scale depending on how many players were present. The fight mechanics themselves were mostly the same, but the bosses would have more heath, defense, stronger attacks, etc. Some mechanics would change, though they would mostly be small. For example instead of the group dealing with 1 add in a normal 8 man, they'd deal with 2 adds.