Waggle, Waggle, Waggle...
Hogan devotes 5 of the 127 pages in his book to squaring the club face in the hands and it was one of the secrets to his ball striking accuracy.
The Vardon grip can create a "doorstop" effect which maxes out travel in the dorsiflexion (cocked sideways) of the back wrist and abduction / ulnar deviation (thumbs down casting action) just before impact where the wrists feel "stuck". Combined with well connected arms to guide the overall shoulder turn path it results in the ability to square the face exactly the same way consistently.
The waggle drill of swinging the club down around the hands to the balls conditions the brain to get the club all the way down and fully extended again as a single long arm-shaft lever just before impact. While doing the waggle and letting the wrists flex down to the max before hitting the ground you check the face alignment to ensure the action is consistent (same angle every time) and it is the desired angle (not necessarily square to target).
The face still needs to be a bit open relative to the target because ideally you want the arm-shaft-face to rejoin as a single "chipping stroke" lever consistently just before the shoulder turn sweep that entire lever unit along the final inch of the shoulder arc swing path. In other words there are really two independent swing arcs for the club head: the small fast one around the hands and the big slower one created with front arm connected to the turning shoulders. The first must bottom out just before the second one pulls club in to turf and ball.
It's an aquired (with practice) skill because the feeling of the wrists getting locked down by the geometry of the grip doesn't feel "right". But in actual fact it's really indentical to what a golfer tries to do intentionally when executing a single lever "chipping" stroke where wrist action is intentionally inhibited to control wrist action.
If you start practicing with wide, no wrist action sweeping chipping stokes you will be able to visualize the bigger shoulder arc and how it forces the club face to open on the way back and close and square on the way down, then close again after impact due to the shoulder arc. It is an important step in understanding the cause and effect of dual-lever swing.
Once you groove the feel of the chipping stroke and can in fact bring face square to ball and hit straight predictable 10- 20 foot chips (no wrist break) just relax the arms (but not the fingers on the grip) and let the backswing start to cock the wrists. Don't try to lift the club with tension in the forearms and hands, just left the force cock and uncock the wrists.
As you increase the length of the backswing and shots you'll notice the more you swing the shoulders back (and how you accelerate while doing that) affects how much the force cocks the wrist. But since the uncocking is action is being controlled by the geometry of the Vardon grip the club should uncock and square the same way but timing will change! The more the force cocks the club up in the hands, the longer it will take for it to come back down to square.
If you do that drill slowly starting with very short backswings and gradually increasing the backswing length you'll come to appreciate how the cocking action is really automatic if you just relax the forearms and let the club pull the front arm straight. You'll feel the stuck feeling of the force pulling the club down in the hands and as it the chipping stroke **how it automatically releases when the force turns turns the toe and front arm along the shoulder swing plane.
In terms of mental swing image if you think "wide full swing chip" without any conscious action in the hands to cock / uncock the club you should find you stop trying to steer with the back hand and the open-square-closing "waggle" action of both the shoulder turn arc and around the hands arc become automatic and more consistent.
On this learning curve the first goal in terms of ball flight is consistent groups not hitting the target. For example when you start from a baseline of a straight 20' chip where you can hit 10 balls to the target then add the variable of wrist action don't try to steer the face to the target just swing the club, let the face open and close, and see where the balls go.
The goal at that point is to hit 10 shots and have them all go the same direction and distance and wind up in a ground, which tells you your mechanics are consistent. Then once you get consistent group adjust timing of shoulder turn (to allow club to square first in hands before impact) to move the group right / left to the desired target.
What you'll realize from the no-wrist break chipping baseline is that the misses, usually to the right, are being caused by a timing issue. Why would chips go straight but pitches right? Because the face hasn't been given time to close in the hands before the shoulder turn sweeps hands and club head past ball. The solution? **Slow down the shoulder turn and give the club time to come around the hands and square WITH THE FRONT ARM before the arm sweeps club through the ball.
Each time you increase the length of the backswing in that drill you'll find it affects timing of the shoulder / hand arcs and the face angle at impact. That's why shots are more consistent if you develop a very consistent full, 3/4 and 1/2 swing action as Dave Pelz suggests in is 4 wedge x 3 swing appoach. If you adopt that approach you'll find you need different shoulder turn tempos to get the club face square in the hands before impact. The longer the backswing the slower the shoulder turn must be to allow the club time to come around the hands consistently squared with the front arm forming that single "chipping" lever again before impact.
The TL;TR is that relaxing the arms so the club cocks/ uncocks itself and slowing down the shoulder turn so the club has time to swing completely around the handd before impact will make ball striking more consistent. You'll know you've slowed the shoulders down too much when you start missing straight left from that starting single lever chipping stroke baseline.