The concept of putter swingweight is simple yet complex.
Swingweight is determined by a variety of factors, including the length of the shaft; the weight of the head, shaft and grip; and the manner in which these various components shape the balance point of the club.
Simply put, however, swingweight is the sensation of how light or heavy the putter feels in your hands (in contrast to the actual fixed weight of the club). And because shaft length is the most common fitting variable at the point of purchase, it is the most common variable affecting a putter’s swingweight.
To understand swingweight, visualize stabbing an apple with a four-foot spear and swinging it around. Now, chop the spear in half and swing it around. Your sensation of control over the apple will change dramatically. The apple will feel lighter and easier to control at the shorter length. In other words, the apple itself still weighs the same (ie: actual head weight), but the swingweight of the spear has been altered.
Expanding on that example, if you have a 350-gram head on a 33-inch putter, it will feel lighter than if the same 350-gram head were on a 35-inch putter.
Most putter manufacturers, however, don’t account for swingweight differences, using the same head weight for all shaft lengths. That doesn’t mean there is something wrong with one or the other, but it does mean that the different lengths will feel differently in terms of their weighting.
Some manufacturers, however, do offer heavier heads in conjunction with shorter shaft lengths. Also, some putters come with interchangeable weights, which give you options for modifying both the actual weight and the swingweight of the club.
There is no “right” answer to swingweight. It comes down to personal preference, and specifically to how the club feels in your hands and how it fits your stroke.
Generally speaking, the phenomenon of swingweight is simply added reason why you should establish your correct putter length before making a putter purchase. Yes, you can shorten or lengthen a putter after it has been purchased, but this will change the swingweight and thus your relationship to that putter.