How Bungee Jumping Works
A bungee jump requires an elastic cord that even when fully stretched does not allow the weight at its end to touch the ground.
With this in mind, the bungee jumper measures the height of the object to which he will attach the cord, factors in his own weight and the velocity he is likely to build up when hurtling toward the ground from the stationary object, and thus derives at the length of cord he needs. The selected cord is then securely attached to the unmoving object. Often, mountaineering gear is used to secure the bungee cord to the object, avoiding a sudden loosening of the knot when the cord pulls tight.
The bungee cord is attached to a specially made ankle harness. The bungee jumper is now securely attached to the stationary object. Although a number of bungee jumping purists deride the use of a backup body harness, commercial bungee jump operators and those just recently introduced to the sport insist on using them. Ankle harnesses have been known to slip free at the moment the bungee cord stretches taut, leaving the jumper to fall to his death. A backup body harness, such as it is familiar to mountain climbers, offers a secondary attachment point for the bungee cord and allows the jumper to survive in case of an ankle harness loss.
After the bungee jumper has securely attached the premeasured cord to the stationary object of his choice---and usually also had someone double-check his calculations with respect to the length of the cord---and also to his ankle harness and backup body harness, he leaps from the jump-off point. The body enters a period of free fall where nothing hampers the building of the velocity. As soon as the rope is fully stretched, the free fall suddenly comes to an end as the rope fully extends. The bungee jumper will still continue to drop further since the bungee rope now stretches, but the speed of the fall is severely curtailed. Depending on the amount of cord elasticity, the jumper may be propelled slightly upward, only to once again fall down. Finally, the movement comes to an end and the jumper hangs on the end of a stretched out rope. He may now be pulled up by an attendant or he may climb up the rope.