Hi Mark H.
It’s interesting that you reference T. Gwynn and F. Thomas. While their credentials speak for themselves, they exhibit swing mechanics that very few other mlb hitters use.
Compared to most other mlb hitters, they have more weight out front. Gwynn used way more hands and wrist than most. Frank Thomas has so much weigh over his front foot that he needs to release his backside to maintain some balance.
Many hitters come up or off their back foot because of the momentum of the bat while they are completing their swing. Some times it is a way to adjust timing when they need to slow down their approach to an off speed offering.
Speaking of Frank. I always wondered how many home runs he would have hit if his swing mechanics were more similar to a McGuire, Sosa, Griffey or Bonds.