I saw a lot of it at our winter tournament. That old back leg straightened up in the backswing. That's cause by the front leg bending too much and/or too much turn. If you turn beyond the toe line, the back leg has to straighten up! That's the way our bodies are built.
But we need to keep the back leg flexed. Think of a tennis player getting ready to serve. Legs are flexed, ready to push off with real power. You sink onto your back leg. And you can't “sink” onto a straight back leg.
This video is Part One to answer the question Jim asked about what the back foot should look like in the forward swing. Well, what it's sup[posed to look is connected to what happened in the backswing. He said his remained flat.
This video about the backswing sets the stage for what should happen in the forward swing. Yes, think of the tennis player ready to serve. Or a baseball pitcher getting ready to throw. And remember: limited turn.
The Surge!