It's the serious concentrating face that jazz musicians make during a particularly good solo
(if you've ever seen any jazz you'll know what I mean)

I was air drumming along to it and got carried away a bit. Ah well, I have very little sense of shame.

Edit: www.youtube.com/watch?v=-J7szWDytCw (random first bit of jazz I found)

Particularly check out Herbie Hancock's face at about the 45 second mark...
(Son of Sam-i-am We're all as mad as hatters here, Fri 6 Feb 2009, 9:47, Reply)