Go-karter put thier hand in front of engine - why?

[vrooom]

Go-karter in this video clip putting thier hand in front of engine at end of straights. why they do it?

http://www.youtube.com/watch?v=bGIGyZ7jPks

[vrooom]

GroundEffect said:

This. It can make a fair difference.

richen ? i thought leaning the mixture would produce more power?