The creance is used as a sort of long leash. This may be anywhere from a few feet long to 50 yards long. The intent is to allow the bird to almost be free-flying while still maintaining control over her, either while she is unsteady and untrusted loose or while she is fat or otherwise in a situation that would invite potential loss. An ideal creance is a strong line that is very light weight. A simple braided (not twisted) Mason's line makes good equipment for a small hawk. Premium Braided Dacron fishing line (50 lb test) also makes a good creance.
There are two very important points which must be stressed regarding the use of a creance. First is that the creance must be used in a safe area - away from trees or other obstacles which a bird could entangle herself on (power lines, brush, fences, etc). Second is that a creance must be anchored to something that will give if the bird does hit the end of the line. Some manuals insist on the falconer's hand (or glove) as the ideal anchor. I attach mine with a carabeener to my hawking bag, which is attached to me. The bag has enough weight to start to slow a bird as she flies, then the leather strap will pull against me. The point here is that a bird should not be anchored to a concrete block or post or other completely stationary object. Once the bird gets flying any distance, even a few yards, she can get up a lot of speed and force. Being stopped very suddenly by an object anchored in the wrong way can seriously injure a bird's legs and even dislocate a joint. Use common sense with any equipment.