This device is intended to produce a loud and sharp sound reliably and reproducibly. It was first used when we required the sound of a breaking twig. Real twigs are not precalibrated, and the only way to tell whether one will give a satisfactory sound is to break it - and even if the experiment turns out to be satisfactory, you'll have to break the twig in another place in the performance, and that might not be so good.
We used a belt. Ours was a rather leathery synthetic belt; we haven't done the obvious experiments, but it seems likely that the material isn't very critical.
The sound is obtained by folding the belt at its centre, and grasping the two ends of the doubled belt with the two strands of the belt in contact. ( This ensures that they will come back into contact when the crack is sounded. )
Without changing the grip, move the hands closer together, so that the two strands part into a "eye".
To sound the crack, pull the hands sharply apart, so that the two belt strands come together quickly. A sharp crack will result.
Practice a few times beforehand. It's very easy, and - in our experience - absolutely reliable. And you can make as many cracks as you want.
We used the crack in one of our services :