The Kong Back-up
This is a good little back-up device for rope access. It has a clever little see-saw camming action that helps decelerate the device on the rope. This absorbs a lot of the shock force generated during a fall.
It very compact and user-friendly. When not in "lock" mode it will follow you up and down a rope. You will need to play around to find the right position of the karabiner for it to follow you down a rope. Once you are in position, lock it off to stop it creeping down the rope.
Here is what Kong have to say (after I fixed some spelling and grammar)-
Safe and simple to use with only one hand! BACK-UP is the revolutionary KONG fall arrester that will follow the operator in both directions and stops the possibility falling. Shifting the special button to the "on" lock mode, it can be used also as a positioning device or a normal locking device. Due to its special manufacturing, the BACK-UP supports also the strongest stress in a semi-dynamic way, avoiding the shocking effects on the ropes caused by the traditional self-locking devices with toothed cams. It works also with NO-CUT LANYARD Ø 12 if used according to EN 567 and UIAA 104.