The Edelrid Ohm II acts to increase rope friction between a belayer and the lead climber. This means that a far lighter belayer can hold a fall from a heavier leader without difficulty. Rather than explaining how it works in words you are probably better off watching the video above.
 
The new and updated version for 2024 makes the Ohm easier to use than ever before. The new swivel joint means you can now clip in any direction to reduce faff. It also means that the belayer can move freely on the ground to find the best belay position without worrying about short-roping you. The new locking system improves safety by preventing accidental opening, and it makes it easier to insert the rope and remove the device when lowering.
 
Details:
Far lower forces are exerted on the belayer
Lowering a heavier person in a controlled manner is far easier
OHM is attached at the first bolt in the safety chain
The device reduces the risk of a collision when falling on the first bolt
Rope handling when belaying a lead climber is not affected (no additional friction when paying out rope)
“Makes the belayer 25 kg heavier”
Minimum belayer weight: 40 kg
For single ropes from 8.9–11.0 mm