I wanna level up to face 7, but i dont understand how the option works, it just locks the yellow bar to max, then what ? it wont level up, am i using it wrong ?

You gain Face XP separate from Triad and Police XP (it's the third tab, right next to those two, in case anyone is having trouble there somehow ), by completing the side-missions around town (the little yellow icons on the map) but when you level it up, it simply gives you whatever bonus that level unlocks, rather than having a choice like Triad and Police do. The yellow bar by your health and minimap is your Face meter. Depending on your Face XP level, you get various advantages when you fill the meter up by dodging blows and generally just doing well in combat. Think of it like a special ability bar that has to be built up by getting a good combo going. It activates automatically, as well.