Back Foot Anchor
To understand this (and the previous) section, it is perhaps best if I compare the one handed backhand to a forehand.
When hitting a forehand the arm is behind the body.
As a consequence, any spin of the hitting shoulder will be coming through in the direction of the hit, and any shoulder rotation through and after contact is helpful to the cause of a powerful stroke and good technique – the above animation of Andy Roddick is a good example of the hitting shoulder’s forehand progress through and into the shot – it powers the racket from behind..
In contrast, when hitting a one handed backhand, the hitting arm is at the front of the body.
Consequently, any shoulder spin through and immediately after contact will pull the racket off course.
This is NOT helpful to the cause of an accurate delivery – it can pull the shot off line and contaminate the throughness.
So on a one handed backhand we want to keep the shoulders (and the body, usually) sideways throughout contact (and beyond), to keep the shot/ball on-line.
So, the ever-classy Hicham Arazi keeps a straight shoulder line in the above sequence and is helping to hold the shoulder line (and his full sideways turn) by using the back foot as a kind of anchor.
If he releases this anchor too soon, it could spin the stroke off course and ruin the directional line (yes, another line!) of his backhand.
In these two animations (below) Sabatini does likewise, and only releases the anchor foot when the force has run its course… and not a moment before.
Again, the force pf the swung/flung racket head pulls the shoulder – not the other way around.