The scales along the flanks of Echis carry raised, toothed keels set at an angle to the body axis. When threatened the snake arranges itself in a shifting figure of coils and slides the opposing loops past each other, and the serrated keels catch and release to make a continuous sizzling rasp. The behaviour is called stridulation, and it is the same physical principle a cricket uses.
The advantage in a desert is water. A conventional hiss expends respiratory moisture; a scale rasp costs almost none, which matters for a small-bodied animal in dry country.
Despite being one of the smallest of the dangerous vipers, the genus Echis is implicated in an enormous share of the world's snakebite deaths, because it is abundant, nocturnal, well camouflaged on bare ground, and quick to strike when trodden near. The precise global burden attributed to the genus remains an estimate built on incomplete national reporting.