Given the small margin for error with the clubface, it is impossible to "lock in" control indefinitely. Our feel for a neutral face fluctuates over time, so we need a means to re-calibrate. The dial is a focus I designed to explore different face positions and their effects. With practice, you could progress to be able to easily fine tune the shape you want.
Clubface relative to path
An essential part of task clarity is to be able to intuitively distinguish between the direction the clubhead is moving on (path) and the angle the face is pointing relative to that line. This exercise is designed to focus on the latter which is the element that most directly determines curvature of ball flight.
If the clubface seems a bit distant, and the loft obscures your view of what square, shut and open are, the dial may help. Focus your attention on the butt of the grip and see how the club can rotate left or right. Remember that for a right handed player (reverse this if your a lefty), at any point in the swing:
Turning the handle clockwise opens the face (fade spin)
Turning it anti-clockwise shuts the face (draw spin)
Start by exploring
Hit some balls with a 7 iron with your focus of attention solely on the butt of the grip. Imagine the arrows pointing clockwise and anti-clockwise. Firstly play with the takeaway only. See what effect turning the dial has on the curvature of the shot. Ignore any mis-strikes, as this is mainly to do with direction and curve.
In theory the more you turn the dial clockwise the more slice spin you will produce and vica versa.
Some or all of the swing
Attending to the takeaway alone may not turn your slice into a draw, however, because unhelpful rotational forces may be playing havoc anywhere in the swing. If I had a life-long slicer whose clubface was open to path at every point of the swing, I'd get them to turn the dial anti-clockwise in takeaway, and keep turning it that way in transition all the way through impact. Turning the dial to the max, may be required initially.
Where does it go wrong?
Golfers who curve the ball too much are probably unaware of the rotational influence they can apply to the golf club. They are also unlikely to be able to pin point the section of their swing where the clubface goes awry the most. Exploring in this way may answer that question.
How this shapes your technique?
If you find that in order to neutralise the clubface you are having to turn the dial in a contrived or seemingly unsustainble way, then it's a sign that perhaps a grip tweak might help you achieve the same effect.
Importantly however this exercise allows you to experience how the club needs to behave to offset your fault, which opens up a clearer pathway to solving it.
Shot-shaping, lobs and bunker shots
If you like this tool as a means to give you greater awareness and control of clubface, you will also be able to use it for shot-shaping and short game.
If you are faced with a lob shot or bunker shot which needs maximum elevation, beyond what your basic technique would provide, then turning the dial clockwise (RH player) would fan the face open, and keep it open through impact, producing added loft and bounce.
Summary
The dial taps into three of the recurring themes in my coaching. It promotes an external focus of attention - on how the tool needs to move to create the desired effect. Because you're not firing instructions at your wrists, for example, you are allowing your body to self-organise. Lastly the exercise is a differential learning tool. So instead of trying to hit a precise position which might be hard to reproduce, you can re-calibrate on your own with the ball flight as your guide.