I am of the opinion that the feet should be in the diagonal stance position always when the pelvis area motion happens, no matter what is the length of the club. Practically, only putter stance would require a square stance, since we do not need any hip joint motion. In order to achieve it the rear foot ball should be in line with the lead foot heel, therefore, sort of diagonally to the target line. Of course, this position is offsetted via different feet position - the lead one is flared while the rear one is square to the target line.
This diagonal stance has one more very important merit, in my opinion. When the downswing progresses, the rotational aspect relocates gradually the CoG to the lead foot heel. At impact, the CoG line should optimally run from the knee joint to the ankle perpendicularily to the ground (letting the knee joint maintain some flex necessary to allow the joint to rotate and guarantees the best possible stability). It is not already at the heel but definitely is closer to the heel than to the toes. The opposite scenario happens with the rear foot where the heel loses its contact with the ground first, maintaining the contact in the area near ball/toe. Now, when we create an imaginary line between those contact points of both feet it can be parallel to the target line only when feet are placed in a diagonal stance position. This creates a necessary torque, since the hips are already open at impact, does not allowing to transfer the CoG too early to the lead heel (causing the lead knee to straighten too early). The same torque does not allow the weight to be left on the rear side too long as well.
Of course, the smaller hip area motion is required, the less diagonal stance is necessary.
Last but not least, the diagonal stance gives a much better base for a human body weight. The bigger surface the base has the easier is to be in balance through the motion and, what is even more important, the easier is to use the ground shear forces comparing to when all body parts are in line with each other. Human feet are very small in relation to the entire body mass that they must keep in balance, especially during such a dynamic motion as the golf swing is.
Here is the diagramme of the Diagonal Stance. The blue line is the target line, the red squares are the address pressure points of both feet and the red lines are linking the pressure points setting the feet (see also the blog part dedicated to feet pressure points where everything is explained in detail):
The above diagramme is, in fact, a corrected version of the original Hogan's stance diagramme published in "5 Lessons":
Watching thousands of Hogan's clips (as well as some other great ballstrikers - Sam Snead, George Knudson and Mac O'Grady come to my mind first) I consider the DS concept as much closer to reality than the latter diagramme. As Mr.Hogan's friend and playing partner, Gardner Dickinson said "[...] he experimented with putting some wrinkles in his left wrist. Ben gradually changed his grip so that his left hand showed about one and a half knuckles at address. At the same time, he let the right hand move more on top of the shaft so that the "V" formed by his thumb and forefinger pointed between his chin and left ear. He then moved the ball forward in his stance, and repositioned his hands at address so that they were about even with the ball, or even slightly behind it with a driver. Finally he assumed a distinctly closed stance at address, even on pitch shots, by withdrawing his right foot from the intended line of flight. Ben aimed with his shoulders, not his feet, so the closed stance had minimal effect on his targeting."
Jackie Burke likes to call the concept "runner's stance" underlining the dynamism of a static position. If you imagine how would a runner stand ready to start the race next words are not necessary at all.
Mr.Hogan's (and other great ballstrikers') stance allowed to locate the downswing vertical axis of rotation in the same place towards the lead ankle joint; the rear foot moves back the longer the club is, makes it finding the bottom of the arc earlier.
A flared lead foot allows to find the limit of rotation of the lead knee faster than the rear one. The knee that cannot rotate further bends inwards what almost automatically makes the weight shift on a lead hip joint when the linear CoG shift happens. If the lead foot is put inward, instead flared, all tension in the knee and hip is lost. Moreover, the sum of small X-factors is being achieved properly when the lead hip "stays" early in the backswing allowing the rear shoulder find its limit without making the backswing too long and loose. Lastly, during the downswing it guarantees a proper correlation of the pelvis position and the impact position. Hips are open at impact, thus, the lead foot should not be square to the target in any case if the aim is to ease the rotation.
Watch Mr.Hogan sets his feet in search for a ground forces and totally balanced motion with the lead foot always flared out and the rear one perpendicular to the target:
Observe how Mr.Hogan seeks for a stable stance that would guarantee the most effective usage of the ground forces. A closed toe line, open hips and square shoulders (in relation to each other), a flared lead while a squared rear foot plus different pressure points for both feet - these are the real keys.
Closed feet, open hips, squarish shoulders = the essence of the diagonal stance. Other words - shoulders are closed in relation to hips but open in relation to feet. Hips are (strongly) open in relation to feet and (less) open in relation to shoulders. Feet are (strongly) closed in relation to hips and (less) closed in relation to shoulders.
Imagine one wants to start a sidewise dynamic physical activity directed at West (assuming one faces North at stance) - can be hammering a nail at the West wall or starting to run to the West, or whatever - the diagonal stance will be chosen best by ones subconscious mind as the most effective one. Hogan knew it and, by an occasion, it appeared to be the best possible stance type to apply D-plane consequences with the fixed ball position (which is a very huge convenience for a golfer if one does not need to think about 14 different ball positions).
Simply ingenious statically and dynamically.
The intention is setting joints the way it is useful both for backswing as well as for downswing. That's why Hogan's diagonal stance is pure genial thing, who knows, maybe the most important thing ever. The rear foot is being settled the way it helps to automate the transition while the lead one to support the vertical axis of downswing rotation the best possible way, i.e. having the mass vector going down through the ankle joint or ensure continuous rotation during the impact zone.
Besides, both feet position match the relationship between hip joints and feet separately in both planes, sagittal and coronal during the setup - on the level of subconscious mind. Rear foot matching backswing while lead matching downswing. It is heavily linked to the process of independent setting of both feet at angles:
