While I agree with Henry that the first push displaces air, subsequent plunges push water. I have actually blown the P-trap off a sink / disposer clearing Thanksgiving potatoe peels. That would be the compression, or push stroke. So there may not be a clear winner here. Plungers are capable of moving material in both directions thereby clearing the clog. Hate to say this, but it reminds me of shaking a bottle of catsup before pouring.