The Edelrid OHM is an assisted breaking resistor, designed to increase rope friction when belaying a heavier leader. Used in conjunction with your normal belay device, the OHM increases the amount of rope friction so a lighter belayer can hold a heavier lead climber in the event of a fall, using significantly less hand breaking force for a safe catch. Furthermore, the OHM makes lowering a heavier climber significantly easier to control, while leaving rope handling during belaying unaffected.