The arms and shoulders create a triangle in the setup, with the putter extended from the hands. The stroke is one of the shoulders doing a combined little rock and rotation around the top of the spine, which moves the arms, hands and putter. For the putter to remain square at impact, the wrists play an important role in that they must remain firm and stable. The problem, as relates to the wrists, is that they are a flex and hinge joint which can break. And they must not break down if you want to make putts.
Most putting instruction relating to grip has always stressed that the forward/top hand, the left for right handers, must be straight or flat at address, with the shaft in line up the left arm. This places the shaft and arm leaning slightly forward and ahead of the ball. Some teachers and players advocate a slight forward press to start the stroke to guarantee this position. To make a good stroke the forward wrist must remain firm and flat to keep the putter face square at impact.
A wrist breakdown will flip the putter ahead of the hands and likely close the face, so the ball is pulled left of the aiming line. The putter also usually de-accelerates when this happens. Sometimes the wrists breakdown or flip because the lower hand over rotates or hits at the ball This causes the top hand to break down also.
The key is: both wrists need to remain firm and even locked down to resist any breakdown. That is why the shoulder movement is the source of the power, not hitting it with the hands. Using the hands as a power source is just inviting them to breakdown. With the shoulders providing the power, the hands just hold on and maintain their position to deliver the putter to the ball.
A good drill to practice the shoulder stroke with hands maintaining their position is to putt using a sand wedge. (Honest!) When putting with a sand wedge, you have to hit the ball with the leading edge in the dead center, the equator of the ball, to have a solid impact and good roll of the ball. Any break, even flinch in the wrists or for that matter shoulder sway or dipping, will cause the wedge to miss the center and the ball will not roll well. Any lowering will cause the leading edge to strike the ball below the equator and the ball will be scooped or popped up. Lifting a little hits the ball above the equator and drives the ball downward into the green. Then it will likely pop or bounce up.
Hitting the equator of the ball with the leading edge produces a smooth and consistent roll. Missing the equator high or low gets no roll and is instant feedback that something broke down, either the wrists or the levelness of the shoulders. Seeing is believing and adds to learning the correct feeling of making a good, level and firm putting stroke.
Practice putting with a sand wedge. It's a great way to fine tune your stroke.
The Surge!