Sudden acceleration and/or deceleration of the brain interrupts its function. Can anyone tell me how a woodpecker escapes knocking himself out?

Yes. They dispensed with brains.

I once witnessed tragedy befall a pair of flickers.They are sadly acerebral. Outside my office widnow at State labs on grounds adjoining Arnold Arboretum was a very large elm on which a large branch had broken off close to the trunk long enough ago that a pair of flickers was able to very laboriously excavate a nest. No sooner had they finished it when a pair of starlings dispossessed them, and ate their eggs. The weak go to the wall. Crime does pay. For a while.
A week later, a pair of squirrels dispossessed the starlings.See pictures of flickers at:
http://www.4to40.com/earth/geography/htm/birdsindex.asp?counter=46