Originally Posted by Gene Horr

This is probably useless information as he's got the transmission off but in my experience you may not need large whacks. Just tapping with a small hammer seems to help. Whether it is actually doing anything or just passing time while the oil/PB Blaster penetrates I've never tested . But spray and tap, spray and tap definitely doesn't hurt and does seem to help.

Another old trick is to hold the wrench or socket firmly in place with one hand and hit/"bump" the handle with the other. Hit it with force but not a lot. If you hurt your hand you are hitting too hard. I've never explored physics behind this but it does work better then just straining your muscles on a seized bolt. It apparently applies more force in that instant than just steady pressure. And is easier on the arms.