I agree that it is not wise to advise a novice to change an element in a full tank. I never like doing that because the insulation gets soaked and remains wet when you want to fire the tank up. In their original message they said that they opened all of the hot water faucets to help the tank drain, if you do that and then open the live cold water on a faucet you will get back pressure down the hot water system. That would explain the phenomenon. How about shutting down the entire house and opening the T&P valve to drain the tank. It doesn't sound like it is a sediment problem because the cold water back pressure wouldn't come out the drain sillcock if it was. It sounds like a vaccum problem. You know, like when you hold your finger over the top of a drinking straw. I also agree with the above crew. Learn to use a voltage meter and a continuity tester or call in a professional.