1988 Lee Trevino Golf Tips for Youngsters - Part 3 - Completion of the Golf Swing
In Part 3 Lee Trevino describes what starts the downswing as well as the importance of the back of the left hand and clubface alignment.
There is some real gold here. Along with what Lee describes in this segment, in his 1976 book "Groove Your Golf Swing My Way" on page 119 he describes the shift in the transition to the downswing: "You should feel as if you’re shifting your weight from the inside of your right foot toward the outside ball of your left foot."
My interpretation is to feel like you are stepping into a throw.
Enjoy!