Being able to prussik up a rope is such an important technique to know, understand and be practised in if you are going alpine mountaineering, which includes glacier travel. In a crevasse rescue scenario, if you find yourself down in a crevasse and un-harmed, you need to be actively trying to get yourself out and the easiest way to do this is by prussiking up a rope. You can't assume that the person who has arrested your fall, and who is now holding your weight in the snow on top of the crevasse, is able to find snow good enough to construct a snow belay, or hasn't dropped their ice axe in the process of holding you and now can't reach the axe to do very much.
Note - If you’re wearing a rucksack at this point then you’ll feel it pulling you over backwards, it’s may be heavy and uncomfortable when you start prussiking. Take the rucksack off and hang it from the rope below you, by simply tying a figure 8 and clipping it into it. The added benefit of doing this is that it keeps the rope tight, and so when you un-weight your foot stirrup and then slide the ‘French’ prussik up the rope, it’s much easier to do one-handed.
Now slide the lower ‘French’ prussik almost up to meet the ‘classic’ prussik, then put a foot in the bottom of the long sling and gently weight it.
Now reach up the rope above you as high as you can and hold it, and at the same time as you pull up, stand in the foot stirrup and push down with your leg. This should allow you to stand up in the foot stirrup with a straight leg. If the foot stirrup is the correct length, then this process will have un-weighted the ‘classic’ prussik, allowing you to slide up the rope so that it comes tight again.
Once again, sit back in the ‘classic’ prussik and you should be 6 inches higher up the rope than you were before.
If the foot stirrup (long sling) is too long, then when you stand up in it, you won’t be able to un-weight the ‘classic’ prussik to then be able to slide it up the rope. A small adjustment is needed and will be different for each person, but either tie a knot part way up the sling to shorten it or if you’re wearing mountain boots, a few wraps around the boot will also adjust the sling length making it shorter.
Continue to stand up in the foot stirrup - un-weighting the ‘classic’ prussik - slide the ‘classic prussik up the rope until tight - put your weight back on the ‘classic’ prussik and sit back - slide the un-weighted ‘French’ prussik up the rope to almost meet the ‘classic’ prussik - stand in the foot stirrup and repeat!
Backing up the prussiks in case of a failure
This is a really important part of the system and needs to be done once you’ve moved up the rope a few metres.
Simply, reach down to the rope below the ‘French’ prussik and pull up a loop. In the loop of rope tie a ‘clove-hitch’ and clip that into a screw-gate karabiner and then into your harness loop.
Now if your ‘classic’ prussik fails, you’ll still be attached and secure on the rope.
As you prussik up the rope (every couple of metres) stop, and just run the distance you’ve travelled through the ‘clove-hitch’ which will keep it about a metre away from the ‘French prussik’ and you secure on the rope. By running the rope through the ‘clove-hitch’ rather than unclipping and re-tying, it’s a much safer and easier process.
The system is exactly the same when using a mechanical camming/friction device such as a Wild Country Ropeman, all you need to do is switch out the ‘classic’ prussik for this device as shown below.