A 'Figure of Eight' friction device is two loops of a special aluminium alloy about 3/4" thick joined together. 'Chunkiness' is important because the heat from the rope passing around the device needs to be disseminated through it - or the heated device will melt its way through the rope if the rapeller stops during a rapid descent. One end of the device clips via a carabiner ("crab") to one's harness, the other hangs free. A rope is passed once or twice around the outer loop then across the throat of the device, between the two loops. The rapeller holds onto the upper part of the rope (if descending backwards - the least terrifying way!) and grips the free end of the rope behind their buttock. If they're using the correct configuration of rope, descender and turns around the device, little more than a strong handshake grip is sufficient to control their descent.

For the record, and despite their popularity, I believe Figure of Eight descenders to be potential killers and won't use one. A guy died on a cliff near my place because his Fo8 snapped across the middle whilst he was descending. When this happens you are left with a useless loop on the rope - and not connected to you, so down you go. I always used a 'whaletail' device instead. These are a block of alloy, rectangular in section and about a foot long. Along one edge are a series of protruding 'whaletails' - lipped wedges - around which the rope passes. Much safer, far greater heat sink capacities and, even if a wedge or two snap off (never heard of it happening), there's still a few left to get you to the bottom. The construction of a whaletail also lets one tie themselves off during a descent if they need a rest or if they need to use both hands. Difficult/suicidal with a Fo8.

As you say, the rope must hang free. A big step to ensuring this is to use a braided rope rather than a 'wound' rope (ie twisted). The rope consists of a core of tiny kevlar or nylon fibres, only thousandths of a millimetre thick, each as long as the rope itself. They are held together by a woven braid casing. (I recall a previous discussion on ropes - in relation to casement windows). The rope itself will be rated not only by its breaking strength, but also by its 'bounce' factor. Climbers require ropes with a high stretchability so they won't be pulled up hard if/when they fall. A rapeller on the other hand doesn't want a bouncy rope, it makes descending unpleasant and difficult to control (hence less safe).

Ropes are carefully coiled in a special way, rather like plaiting, and packed into bags. Done properly, the rope will undo itself without kinks or snags if it (and its bag) are tossed from the anchor point at the top of the descent. Looping them like a cowboy's lariat is guaranteed to induce loops and tangles and therefore not regarded as a long term practice in the field. They should also never touch the ground or be trod upon - the tiny particles of dirt will chop their way through the braided wrapping.

Rapelling with only a rope is achieved by wrapping the rope around your limbs and trunk and using clothing as padding. This is highly ineffective with a braided rope (they are made to operate with a low coefficient of friction and hence extremely difficult to grip), and more easily achieved with a wound rope. Only for emergencies in my opinion.

BTW - The bottom end of the descent rope should have a large knot tied in the end so the person doesn't rappel off into space. A sky diver friend of mine has however rappelled off a rope at 10,000' from an aircraft!

stales