This is a neat way of making an extendable safety line that I picked up off a friend of a friend I was climbing with in Yosemite (and who happened to teach rope and kayak rescue for a living). He gave me a spare one to try out, which I later carefully dismantled to learn how it all went together.
There are several advantages to this setup. Firstly, the prusik knot will slip under a large shock load, thus dampening the impact. Secondly, the material itself is dynamic, which also reduces the shock load. Last but not least, prusik cord is cheap and readily available.
The importance of a dynamic safety line is made clear here and here. “Even a 60 cm fall-factor 1 fall on to an open Dyneema® sling can generate enough impact force (16.7 kN [= deadly])”
Edit: here’s another excellent study of the purcell including cool testing videos.
Now back to the knot. You will need about 4 metres of 6 or 7 mm prusik cord. Double this over into a large bight with one tail end about 60 cm longer than the other (this will be the piece you tie into at the end). Then tie a classic prusik knot at the end of the bight.
Next you thread the tails through the prusik knot.
Now we tie another loop in the tail end of the cord which we will girth hitch around our harness. To do this firstly tie a figure of 8 knot near the end of the shorter tail.
Then re-thread the figure of 8 with the longer tail forming the tie in loop (this is the same principle as when tying in to the end of a rope). Note that to form this loop you could also use a double or triple fishermans knot.
That’s all folks, now girth hitch that sucker to your harness, and you’re off!
A double figure of 8 knot where the one tail is then re-threaded is called a “Frost 8”. It can also be tied in the one fell swoop as follows. (Although I find the method above more instructive because it parallels the single re-threaded figure of 8 used when tying into the end of a rope.)