they work by points. The more points you get, the faster you unlock them. However, if everyone in your squad is wiped out at the same time, or you are all dead at the same time, you lose them, and have to earn them back. If your the last man in your squad alive, it will pop up on your screen "Last man standing" and you wanna make sure you stay alive until another squad mate spawns in so you dont lose your perks.
