It is tied around a climbing rope and allows the climber to ascend the rope, either by using another prussic loop, or by using a single prussic loop and a mechanical ascender, sometimes a belay devices is used in place of the mechanical ascender, if a belay device is used it will an "auto locking*" device.
- auto locking belay devices may in some circumstances fail, for this reason it is important to have a fail safe, so that in the event the device fails, the climber does not hit the floor.
Step 1: The Way It Works.
Step 2: Larks Foot.
Because you are already tying this with a loop, all that you need to do is take part of the rope, and pass both strands between the loop, then pull down, and you have created a Larks foot.
As well as doing this with climbing rope, I have (as per request, always like to get feedback) re-shot some photos, with a carabiner and access cord. Whilst they are exactly the same thing, there is no point what so ever using this knot with a carabiner. It does give a better colour contrast which I think is the main issue.
Step 3: Turning a Larks Foot Into a Two Wrap Prussic.
Create a gap in the middle of the larks foot.
Pass the end through though the gap that you have created.
Tighten down and tidy it up if needs be.
I have reshot this sequence too, using the carabiner for clarity. Remember you want to be doing this with A rope rather than a biner, becuase well, this biner, even though one of the largest on the market is only 120MM high.
Step 4: Finish.
You can now move this knot up and down the rope by moving it up and down, but if you put force on the tail of it, the knot will clamp to the rope and will be stuck fast.