When I was a "Yut", I remember my Dad doing the same thing in his '74 Ranchero. He used to use it to push the snow around in our driveway when it got deep. Sometimes when he would back up and stop, then he would put his left foot on the brake, give it a bit of gas, then go. I asked him why he was doing that. He said that because a posi does slip that he was preloading the rear and that it usually helped the posi grab on both sides. He was right. Sometimes when he started forward and it was slipping, He'd hit the brake then I could feel the truck shimmy a little side to side and start plowing thru the snow. Years later, I'm in my '94 T'bird. It's got a LSD in it. One day after a snow storm I was out for a ride. Went to turn around and got stuck part way in a ditch. The posi just didn't seem to be grabbing. Then I remembered what my Dad did. So, I held the brake for a moment, gave it some gas and Behold! POSI!