They attach an electrode up top (say by your knee or ankle) and then measure a zap through to a spot on your foot, to measure how fast the impulses go through the tarsal tunnel. It feels like a shock and your foot may jump a bit. If your foot is very sore the day you have to go to the test, you may want to take a tylenol ahead of time but I doubt very much you'd need it.
They do a series of these shocks, and then compare your results to what most people have. Make sure they do both feet to compare.
I had this done in August, and I have to have it re done along with my wrists.
Like I said, it's not any fun - but it really isn't a big deal (I am not dreading it or anything), the tech told me to relax as much as possible in order to make the test easier....so do your deep breathing and hope that this helps with your diagnosis.