In reply to Ned:
Now that is an explanation I can understand - thanks!
I've been struggling to identify what part of me would give way if I wasn't winging and why / how it works. My confusion was that it is clearly the fingers that cave in but the winging seems to be utilising big muscles in the shoulder/back and I wondered if the was a postural advantage going on but your explanation of supporting tired fingers fits the bill perfectly.
So as a training pointer, what you need to train more if you need to wing is finger strength, if you wing when fatigued then endurance, if you wing to get a particular move done when not tired probably max strength?