Was going to put this as a comment in reply to Kevin Howell, but it will be longer than a comment will probably allow.

Speaking from a physiology standpoint, it is going to probably act much the same as the muscles do for the leg. Your quadriceps on the front bring your leg from a bent angle to a straight angle, and the hamstrings in the back do the opposite.

So I would surmise that (surgically rearranged or not), there are opposing sets of muscle/tendon complexes (tendons connect muscle to bone, you have anchoring tendons at each end of a muscle). One set relaxes as the other contracts, and the claws slide out. To retract, the opposing muscle contracts as the first one relaxes.

This would also give a good explanation of how Wolverine is able to control the speed and individuality of the claws, much like you can make a slow or fast fist, or extend different fingers apart from each other. (With some exception in various positions for the middle and ring fingers as they share a tendon).