hey... what's this knob do? 

http://galleries.burningman.org/filestore/4/1/6/4/0_6be29cf20ce4db4/41640pre_1bb7ac8f34cbece.jpg?v=2013-08-23+20%3A32%3A41

hey... what's this knob do?

http://galleries.burningman.org/filestore/4/1/6/4/0_6be29cf20ce4db4/41640pre_1bb7ac8f34cbece.jpg?v=2013-08-23+20%3A32%3A41

hey... what's this knob do?