ass or brains is asked while(or before) a flipping the coin, you ask ass or brains, and if the coin lands on "heads" its brains.. and if it lands on "tails" its brains
pull your brains out of your ass means you need to pay attention and stop ignoring problems around you
the boss was not happy with his employeesslacking off and not paying attention to problems around them the boss told employees i need you guys to pull your brains out of your ass start paying attention and stop ignoring problems around you