I've had the numb hands and fingers, usually worse first thing in the morning if I am poling constantly, I just rest the pole on the ground and shake my arms and hands loosely by my side for 30 seconds and it goes away.
My problems in that area started when I was a decorator, was using a high pressure water jetting unit (approx 10,000 psi) and you had to squeeze the trigger so hard to get maximum pressure.
Had fingers like sausages by the time I'd finished...used it on and off quite regularly for a couple of years and have suffered ever since
What can make things worse for someone using WFP is that some of the poles can't be reduced, so you are using and lifting all the sections all the time.
One of the advantages of the Unger poles is that you only need to use the poles you have to, and simply add or take off those that you don't need, it is a big advantage to pole systems that have the pole hose on the outside of the pole and not up through the pole.
And of course as Trevor has said...change hands frequently!! After a while you will do it without thinking about it.
Ian