How do 'hand clap' switches work?
The most famous example of this was &quot;The Clapper&quot;.
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/