OpenBarbell V2 was designed to be better in every way so we felt the way you attached the device to the bar needed an upgrade as well. The magnetic roller hook is a quick and easy easy way to attach the string to the bar.
The roller hook also solves a simple issue, bars aren’t static during a lift. Both the bar and collar can rotate taking the string along with it, causing not only a slight annoyance but a loss in accuracy as well. When every hundredth of a meter per second counts, it’s not ideal.
The roller hook can only be used for lifts that don’t hit the ground. The vibration from dropping the weight forcefully, such as in a deadlift or dropped olympic lift, can break the magnetic attachment between the hook and bar. For those lifts you can use the included velcro strap.