Would you agree with this concept of extension: "hit the ball as long as possible and extend through the hitting area after contact"
Depends on the swing being employed..... I understand the cue, but it could just reinforce pushing through contact. If you are whipping through contact, then its a non-issue. Actually, if you are whipping through contact, you can't help it. Objectively, I personally don't like the cue of hitting the ball as long as possible. But sometimes it can work well with a player. You never know what will make a player get it. Sometimes its the upstream stuff, sometimes the downstream stuff. I think the more you teach a player, the more their knowledge base gets, the further upstream you can go and the more they will understand why upstream is the most important. (To me, downstream is all band aids.)