[Yoda]

As a coach, I don't really like seeing the putter come up through impact. This seems to be the reaction of a bent left wrist.

Right you are, Spike.

In uncompensated Golf Stroke Geometry, the Left Shoulder serves as the Center of the Left Arm Swing (and, given the Flat Left Wrist, the Clubhead Arc). Therefore, the Low Point of the Stroke occurs directly opposite the Shoulder (when the Radius of the Stroke -- the Left Arm and Club -- points directly to the ground).

Alternatively, the Bending Left Wrist can replace the Left Shoulder as the Center of the Clubhead Arc. And when the Wrist Bend happens through Impact, the Clubhead has no choice but to come up.