He's holding a clarinet bell (that's the flared doodad at the bottom of a clarinet), and he's inserting into the end of the bassoon.
The bassoon has a designed pitch range down to low Bb. On this particular song, the composer wrote a descending scale down to low A, i.e. out of range. So what do you do? Extend the length of the tube. It happens, by accident, that a clarinet bell fits into the tube and is just the right length to play low A in tune. So the guy holding the bell (who is playing a flute part) is watching the bassoonist's music (you can see his eyes on it) and plunks the bell into the bassoon at exactly the right moment (split second timing) to play the problem note. Then he immediately removes it and goes and sits down.
So you ask, why does a flautist have all this extra time, and why am I taking pictures instead of playing my instrument (clarinet in this case). Because this particular song had reduced scoring, so most of the wind section was not playing. So not only was it technically possible to fix the out-of-range problem with a trick, but the player next to him was available with his eyes and hands to perform it. A rare trick indeed!
So where did the clarinet bell come from, you ask? The bassoonist also plays clarinet (plus flute, you can see it in the picture), and he brought his own bell from home.
-- tsreknor
Trumpeters use metalic inserts to raise the pitch of the instrument. I guess, the guy in the background put in the insert, so the air came out with higher speed producing higher tone on the bassoon.
