The Sargent Door Closer Hold Open Arm 25-H is known as a Friction Hold Open Arm. As the arm rotates it tightens the two plates together until they can not turn any more, and holding the door open. To close you just pull the door and it will "unscrew". The hold open position is adjustable by tightening or loosening the larger nut.
Sargent part # 25-H