Both of mine were to relive the post surgery pain on my ankle. They started off by taking a bendable needle that was about 10 inches long with a small electrical charge and sticking it in the front of your upper leg so they can find the nerve that's needed. Then they inject this stuff that isolates and completely numbs the nerves in your lower leg.
When I had the second one done, I got out of surgery and had no pain what so ever for the first day and a half or so cause I had no feeling.