Dear James,
In my view, the best defense against second guessing the aim of the putter (which is also the "line" of the putt) is careful, accurate aiming to begin with plus the realization that aiming is a staged process that results in only one aim, while a second guess of aim is obviously NOT based on any process. Hence, one should never acknowledge an "aim" second guess as meaningful for reaction -- just ignore it totally unless you recycle the whole aiming process.
The best defense against second guessing the distance or touch for the stroke is a reliance upon tempo plus targeting for the size of the stroke (and hence the putter head speed at impact). If there is a second guessing about touch, then you can simply relook along the line (with visualization) to recapture a sense of the appropriate touch. This leaves the aim unaltered.
In my view, there is no such thing as second guessing the stroke movement pattern itself -- every putt rolls the ball straight the way the putter face is aimed with solid impact and good tempo. There may be a discomfort in the setup for some reason, but even so, the making of the stroke can still be performed accurately out of the normal positioning at setup, or can be made with slight mid-stroke corrrections after detecting problems in the backstroke or early in the downstroke.
The common problem for most golfers really seems to stem from not completing the aiming and accepting the aim as the best you can do before moving to the stroke itself. Because of this, aiming continues or smears over into the making of the stroke. Reacting to a second guess at this time is often done without the golfer clearly knowing what is prompting the urge to do something different at the last minute -- aim, touch / distance, stroke.
Many instructors speak about "committing" to a line, but this is an odd way of thinking about it. The aiming is a process of reading the putt, selecting a start line for the putt, and aiming the putter accurately on that line. The steps in the performance of this process have to be done carefully and accurately in order to aim correctly on the chosen line. The process results in only a single aim of the putter, and a single line -- so there is NO alternative aim to consider. The notion of "committing" to a line necessarily implies that there is some other line that might be chosen instead, but there is not any such alternative. Hence, talking of "committing' to the line doesn't make any sense -- you MUST commit because it's the ONLY aim available.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
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