The French prusik, or autoblock, is another in the family of slide-and-grip knots that includes the standard Prusik knot and the Klemheist. The main difference between the French prusik and the other ones is that the French prusik can be moved while weighted.
This can be useful, although it also makes the knot more susceptible to slipping than the other ones. As a result, it is better used as a backup brake hand below the descender, rather than an abseil backup above the descender.
The knot must be tied with a rope of diameter less than the main rope, as the effectiveness of the knot is reduced the closer the two ropes are in size. 6mm cord around double 9mm abseiling rope is usually sufficient.
The number of turns can be adjusted to suit the conditions. More turns means greater friction.
You need a relatively short length of accessory cord to tie this knot. Too long a cord will introduce too much slack into the system. We will assume that the cord has already been tied into a loop.
Start by taking the end of the cord furthest from the knot and passing the loop around the rope as shown.
|Pass the cord around the rope again...|
|... and again...|
|... and once more.|
|Clip a carabiner through both ends of the loop.|
|The finished knot.|