The prusik hitch is the friction hitch that every rope guerrilla should know. It’s tied with a loop of accessory cord and serves as a “soft” rope grab, gripping when under tension but able to slide when the load is released. It grabs in either direction and makes a great point of connection when working on a horizontal safety or restraint line.
Use the prusik hitch in a variety of applications; hauling, rescue, banner rigging, ascent/descent – the list goes on. The prusik hitch (developed in 1931) remains a tried and true alternative to the mechanical rope grabs to which it gave origin. It’s simplicity, many functions, low weight, and low cost make it irreplaceable.
One method for using a prusik hitch as an autoblock or backup for a rappel.
Benefits:
- Simple
- Easy to tie
- Multi-directional
- Multi-purpose
Learn to tie the prusik hitch here.
ROPE GUERRILLA 
Probably better to put it below the device, a panicked rappeller will likely clutch the rope, tending the prussik and defeating the purpose. Plus, even if it fails, the knot is likely to jam in the device and arrest the fall.