If we agree that the stance should be so that at address the weight is centred between the feet at a 50-50 ratio and that the pressure in the feet should be slightly forward (balls of the feet or arches). When you make the backswing there should be a certain amount of spine-tilt away from or behind the ball, as your arms move out and wide then your centre of gravity is shifted right with the motion of the arms and their weight being distributed to the right and outside the body's centre line. As a result you would either roll your left foot in or, in some cases, lift the left heel.
All current teachings is that the downswing starts from the ground up, if that's agreed then the first movement should be to replant the left foot (roll left foot back to flat) or plant the heel and swing arms,club and subsequently c.o.g back to the left. The picture shows (I think) an exagerration for visual impact, in reality I think it would be a lot less, eg Tom Watson in the other picture.
I'm no expert, I did get an awful lot out of Shaun Clement's website www.clemshawgolf.com - see the vids he has there on left heel planting - they make sense to me and have certainly improved my awareness - if not my game.