When you use your Elder Charm, it rerolls the loot. If a leather piece and a str trinket drop, and you use your Charm, you can get any piece for your class. The bosses current dropped loot does not matter.
As others have said, only the loot that regularly drops from the boss can be rerolled. If you win, you'll get an item appropriate for your class, but not necessarily your spec. You have roughly 30 seconds to decide if you want to do the roll, so if you're in a master looter run and the master looter is dead, rez them fast. You get up to 3 Elder Charms a week and can stack up to 10 on your currency tab.