With my HBAC, I also pushed really hard. I couldn't help it! The urge is like that which takes over when you projectile vomit (sorry gross) because you have no choice in the matter, you just relax and let your body take over, and try to get through the ride. I never gave it any thought. My midwife never told me when to push or not to push, how to breathe, or anything else, just followed silently trailing behind me with catch cloths as I made my way up the stairs to the bedroom, naked and dripping from the labor pool, to where I wanted to be, pausing every few steps to push and holler.
And he was a "big baby" for sure, over 10 lbs and came out with nuchal arm. But the sac didn't ever break til after his head was out, so I think that helped. Even still it was intense, but no one ever suggested I should do anything but labor the way my instincts moved me to, and everything went fine. hope this helps.