The OHM is an assisted-braking resistor to increase friction. It's attached at the first bolt in the safety chain using a quickdraw and the rope runs through it. In the event of a fall, the OHM increases the amount of rope friction so that a lighter belayer can hold a heavier lead climber, regardless of which belay device they are using. The belayer needs significantly less hand braking force to arrest a fall. Lowering a heavier climber is much easier to control. Rope handling when belaying a lead climber is not affected (no addictional friction when paying out rope.) Should a lead climber fall at the first bolt, the OHM significantly reduces the risk of a possible ground fall.