The Prusik grips under load and catches you when abseiling. When it holds, when it does not, and why material, length and wraps decide the outcome.
The Prusik is a friction hitch made from cord that slides while you push it and grips as soon as it is loaded. When abseiling it sits as a back-up between rope and leg loop: if you let go of the brake strand, it should hold you. The honest answer to "Is that enough?": only if material, length and wraps are right and you have tested it before going.
How serious this is shows in two abseiling accidents on DAV courses that safety research investigated. In the second case the Prusik was laid and attached correctly, yet it did not "bite". The fall went about seven metres to the ground [1]. The testers found the cause in pull tests: a relatively stiff 6 mm cord failed several times as a back-up [1].
The DAV also says: a back-up is in principle always a safety bonus [1]. That is why this topic is not for half knowledge and not for self-study. You learn it in a course, with a coach, on the ground and in the load test.
The Prusik only catches what you built correctly for it.
Entry criteria (practice standard):
The Prusik grips through turns of cord around both rope strands leading to the ground. It grips under load and can be released when unloaded, for example by lifting the leg when attached to the leg loop [1]. It is a brake with delay, not a knot that locks instantly.
Milestone: You explain to your coach when the hitch grips and when it slides freely.
The thinner the rope, the greater the risk that the back-up does not grip [1]. Stiff cords, for example with a Kevlar core, responded clearly worse than soft cords with a Dyneema core [1]. The usual Prusik has two wraps; if it does not grip, one more wrap usually helps. For single strands or very thin ropes the DAV recommends more wraps [1].
Milestone: In the course you test different cords on the ground and recognise which grips and which does not.
The strength of untidily laid Prusiks varies immensely [1]. It must be laid cleanly so the wraps lie parallel without crossings. What "clean" means you see on the rope with your coach.
Milestone: Your coach sees you lay five hitches without correcting one wrap.
A Prusik that is too long is pushed along in front of the abseil device and is useless [2]. The DAV advises keeping it short so that it does not butt up against the device and can therefore always grip [1]. Your coach shows you the right length for your abseil method.
Milestone: You explain why a too-long Prusik is dangerous and show the right length.
Before every abseil the system is loaded while you are still self-belayed: pull abseil device and back-up towards the stance, sit into the set-up, check that the device takes the load and that the back-up grips the rope [1]. If you have to help by hand during this, it should be improved, usually one more wrap helps [1].
Milestone: You do the test in the course every time and say out loud what is being checked.
When abseiling the Prusik is only one part of the chain. In both DAV cases the cause was not the hitch alone, but a set-up that was not loaded before the self-belay was unclipped [1].
This guide explains and motivates, it does not replace supervised training. The DAV itself writes that its article is not an instruction for correct abseiling [1]. Learn it in a course.
In the course, deliberately try out how your Prusik reacts to a load on the ground, with two wraps, with three and with different cords. Anyone who has seen a stiff Prusik slide checks their material differently afterwards. That is course practice supported by the DAV test [1].
As a back-up: if you let go of the brake strand, it should hold you. The DAV calls a back-up in principle always a safety bonus [1].
Soft cords with a Dyneema core respond clearly better than stiff cords with a Kevlar core. DAV pull tests showed that stiffer, thicker cords failed several times as a back-up [1].
Common reasons: rope too thin, cord too stiff, too few wraps, laid badly or too long so it butts against the device. Usually one more wrap helps [1][2].
No. A hitch that depends on the millimetre and the wrap is learned on the rope with a coach, not from a video. Your test in the course is the proof.
So short that it does not butt against the abseil device. A too-long Prusik is pushed ahead of the device and is useless [2].
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