Someone described something similar as "like sneezing". You know when a sneeze is coming on, and you can delay it, but it's hard or impossible to actually stop the sneeze. Is that a good analogy?
Prior to reading that analogy I just could not wrap my head around the "something they don't want to do but not actually involuntary."
The desire to scratch an itch explains it perfectly.
Then again I sneeze when ever I look up at a clear blue sky so it might just be my wiring. :)
Breathing in through my nose intensifies it.