Billy Casper has written that his wristy-style putting depends on returning the putterface to vertical just before impact, and that practicing this one point should receive a great deal of attention. For a wrist-putter, it is obviously important, but perhaps it is not so obvious that it is equally important for any other putting style as well.
Returning the putterface to vertical just ahead of impact is not inconsistent with keeping the left wrist firm or even with forward pressing and leading through the stroke with the back of the left hand ahead of the putterface (depending on the loft of your putter). The fundamental point is that the pivot point of the stroke (in your neck) cannot be seriously ahead of or behind the putterface during impact. This is precisely the same dynamic in the full swing, and the same constellation of related rubrics apply in putting: "stay behind the ball"; "hit into a firm left side"; "hit into a wall."
Some Theory
A straight stroke is one that sends the ball where you intend for it to go, preferably without sidespin, bouncing, excessive skidding, or hopping. The safest, most direct way to make such a stroke is for the putterhead to be "square" to the putt line during impact and moving such that the putter's sweetspot travels along this line and through the ball's sweetspot AND with the trajectory of the putter's sweetspot staying pretty level and parallel to the surface. The notion of what a "square" putterface is usually starts and stops with "perpendicular" to the putt line -- aimed at the target. But this accounts for only two of the three dimensions of the orientation: one also needs to be careful about "squaring up" the face vertically.
With the exception of a few putter designs (Teardrop's Roll-Face, and similar putters), the faces of putters are all flat. Since the ball is a sphere, the contact between a flat planar surface and a sphere occurs in only one point. If the desire is to have the putter's sweetspot moving level through one spot on the back of the ball's equator through the ball's center of gravity, as it should be ideally or nearly so, one should notice that it this is physically impossible UNLESS the putterface is vertical at impact.
Any forward or backward tilt in the face at impact means that the point of contact must be either higher (face tilted top edge forward) or lower (face tilted top edge backward) than the ball's equator. When the impact point is above or below the equator, the line from that spot through the sweetspot of the ball is necessarily tilted out of horizontal. A "solid" putt in such a case is one in which the putter's sweetspot follows this same tilted line from the impact point through the ball's center.
Trajectories tilted down cause some pinching of the ball into the spongy turf with rebound and bouncing back out. Trajectories tilted up cause launching of the ball and bounding. Any putter sweetspot trajectory other than level through the equator and the ball's center will also be at least a little thinner and cause some degree of bouncing or bounding, including a level trajectory above or below the ball's center.
To the extent your putter has built-in loft (usually about 3 degrees), you should simply ignore it and live with it. If the loft is causing problems, you should get another putter rather than try to adjust your stroke dynamics. The putter is at least "stable," whereas your stroke is not.
While there is a range of severity here, so that it is possible to get away with some tilting of the face or trajectory without serious problems, in general it is best to stay within this safe zone by aiming for impact without any tilt. As soon as you intentionally try to ADD tilt of a specific sort, hunting for
some mythical "true roll," you create problems for your putting that can only be overcome consistently, if at all, by tons of practice and experience and counting on equisite timing and fine-motor control to be there for you every day you play. Not likely!
Keep a Simple Plan for Impact by Returning the Face to
Vertical
Any sort of impact other than with a truly "square" face is strictly a matter of finesse. The gain in aesthetics from a tilted-impact stroke is difficult to learn and maintain, not usually worth the price in terms of added accuracy and precision, and is far too susceptible to breakdown or to slightly-off execution. In fact, the ONLY sort of impact that is accompanied by reliable and readily reproducible body positions is this "square" impact.
Target the Gap at Address
At address, there is a small gap between the ball and the putterface. If you "target" this gap as the place for resquaring the face vertically, you will be much more likely to keep the stroke pivot in your neck area aligned.
A Good Kinetic Image
Ben Crenshaw and others have described the putt stroke as "definitely a sweeping motion." Maybe so, but there is a danger in thinking this way. There is a "sweeping to" and a "sweeping away." There ought to be a "sweeping through" as well, but this implies the face is vertical only for the briefest moment. Timing this so that moment is only at impact is quite a trick! Don't count on it. The sweeping image is not very helpful.
A better image is to think of a combination of a swing-blading low and level move into impact with a hammering or nailing action at the moment of impact. The classic tip, which dates back at least to Walter Travis at the beginning of the twentieth century and was brought forward by Bobby Jones and others since, is to putt as if you are nailing a tack in the back of the ball. Obviously, this hammering or nailing action requires the face of the putterhead / hammerhead to be vertical at impact and to be moving on a trajectory that drives the spike of the tack straight through the center of the ball and along the line of the putt.
A modification of this image is to imagine a very long, thin, straight nail through the ball extending all the way to the target, with its sharp tip poised to enter a soft wooden pole in sticking up out of the center of the target, with the head of the nail extending behind the ball a quarter of an inch. Your objective is to putt the ball so it drives the head flush with the ball and imbeds the tip in the target. This is simply for orienting the stroke, and not for the force of the blow.
The swing-blading only needs to be low and level in the half a foot or so before impact through a similar distance past the ball, so don't worry too much if the backstroke carries the putterhead up some. It's not too bad so long as the rising occurs without your arms or hands exacerbating the rising.
The transition from the "swing-blading" movement to the hammering at the last instant requires tight management of the pivot of the stroke in the neck. This is where the moment of truth is. Done properly, it feels almost exactly like a crisp iron shot. Bobby Locke used to illustrate this point by the sound of his putts, with solid impact giving something like a "pock" sound with putts with the face out of square giving more of a "tuck" sound. Solid impact usually sounds closer to a "click" than nonsolid impact (higher or brighter pitch and a bit fuller or distinct sound).
Make This Part of Your Game
On the practice green, make a few putts to nowhere in particular just to observe face orientation at impact. Watch the gap between the ball and the blade before starting the stroke and keep your pivot from moving laterally during the stroke. It's important to square the face up both in terms of it's online "aim" and also in terms of its vertical presentation to the back of the ball. Even if your actual impact has a slightly tilted face or a trajectory other than level through the ball's center, try to square the face vertically just before impact will increase your consistency and make you more accurate and precise.
Swing-blade your hammer into the tack for sound and good-sounding putts.
