The condition of each prc set is describet in the item description you see in your inventory. The thing with why its often used on the back bar is, that you cam run an 2piece weapon on your frontbr and still have two 5p piece bonuses.
For example:
Maelstrom flame staff FB
5pieces armor of any set
3pieces armor/jewelery of burning spell wave
flamestaff of burning spellwave BB
So each time you swap on your BB you have the chance to proc extra spell damage which will be active for 8 seconds regardles of BB or FB.
Edit: the proc condition is not bound on any bar, you just need the required number of set pieces active.