How do 'hand clap' switches work?

The most famous example of this was "The Clapper".

See the patents for the details: 1 and 2

https://www.instructables.com/id/how-773/step1/Clapper-Circuit-Diagram/

_{(source: instructables.com)}

Here's a recent hackaday post on achieving the same thing: http://hackaday.com/2011/02/07/diy-clapper/

What's typical for clapping hands is that it's a sudden short, but strong sound. (The sound of a barking dog may look somewhat similar, but the peaks will be longer.) A possible way to detect it is to trigger an MMV with the input signal if it's higher than a certain threshold, and see if this high level is still present when the MMV times out. If it isn't it was probably clapping hands. You'll have to experiment with threshold level and MMV time.

Not filed for patent :-)