Well, Do you happen to know what the pupose of a relief valve does, By opening the relief valve you do 2 things:
1: Regulate the atmospheric pressure in the tank to allow it to drain, if this is not done the tank air locks and it will not drain, it will literally take hours to drain a tank.
2: Verifies that the relief valve is functioning. Water heaters are a time bomb without a relief valve, or even a faulty one. If the valve is faulty (sticks open and ; or is seized) THIS IS VERY BAD, change the valve asap. The relief valve is one of the most important valves in the plumbing industry, You wont believe the destruction that these can cause if they fail!!!
Rocking the tank.
The bottom of the tanks are convex, not flat allowing water to pool in small crevices, when water freezes it WILL exert over 1000 psi on its surroundings and WILL CRACK THE TANK. GENTLY rocking the tank will allow the home owner to hear any water left in the tank The tanks will rock, there will be enough "play" in the water lines to allow it to do so, but just be easy on it done slam it around.