you loosen it 1/4 turn and it has a check ball in it so it releases the brake fluid and shut off when the pressure is released..I gravity bleed while nursing a beer then finish it up with the speed bleeders. I bought the first set from a speed shop a few years ago then they were just new up here so they cost me close to $40 for the 4 but now they are at the local parts store on the help rack for about $24 for 4