The stone knot is used to isolate two rappel strands so that rappellers can rappel down either strand independently. There are four different variations of the stone knot:
The stone knot allows rappellers to select whether they want to rappel single strand or double strand. It can speed things up if you have a large group and you want to overlap the getting-on-rappel and getting-off-rappel stages.
The stone knot does not allow you to lower a stuck rappeller.
The last one down needs to remove the stone knot before rappelling down.