[Michael Romanos]

Peggy has the right answer though I must say that heel or back of leg nipping is a very common Corgi trait and they of course do this with cattle and sheep to good effect. This kind of action woiud also be common with some other breeds who are working (farm) breeds, but it is not such a common trait among dogs of any description. With one of my Corgis I found it amusing and never did try and stop him from his odd little grab/nip when he objected to me going out and leaving him behind.
Like Peggy says, treat the act as though it really hurts and say a loud, longish OUCH and when your Corgi lets go, stops, runs off etc, give praise. That ought to work after awhile. Report back.