Hi, there and welcome to the forum. Your answer is easy. When you pull on the skin of your arm to make it taut you change the pressure and maybe position of the nerve that is actually acting up on the outside.
The nerves are likely just fine in your arm. As you know, in MS, the damage is in the central nervous system. The problem that you are having in your arm is likely due to a problem in the spinal cord.
The numbness you are having is from a paresthesia. This is a faulty signal in the pathway from the arm to the brain where you actually can "sense" what the nerve is saying. As that signal moves to the spinal cord, it got damaged by MS and is sending jibberish to the brain. When you mess with the arm like you describe, the jibberish changes a little.
I recommend that you read the arcticle we have in the Health Pages called "Paresthesias - Things that Go Buzz in the Night"
http://www.medhelp.org/health_pages/Multiple-Sclerosis/Paresthesias---Things-That-Go-BUZZ-in-the-Night/show/378?cid=36
That might help explain how the attacks on the myelin cause such weird sensations.
And, you're right. We often hear about exacerbations after surgery.
Hope this helps.
Quix