The shaft definitely bends. It also rotates. One source of the force that bends the shaft is the inertia of the club head vs the acceleration of the club at the beginning of the downswing. During the downswing the club head acceleration overtakes the shaft acceleration (at least for a good golf swing) and this source goes away. Another source of the force that bends the shaft is the distance between the center of mass of the club head vs the connection to the shaft. As a result of this source the club head will wind up leading the shaft into impact and more so with wood club head because of the larger offset. The club head also 'droops' (the shaft is bent towards the ground) at impact as a result of this source.
It's the second source that makes it valuable to spine align all of your shafts and is part of the reason matching shaft to swing and driver head is VERY important if you want to maximize distance since the the club head lead changes the loft at impact.
You may find this article
by Tom Wishon interesting.