shaft extender

Ever since January 1, 2016, when the USGA and R&A banned anchoring the putter to the body, users of long and belly putters began to look for new ways to make the stroke simpler and stress-free. One such adopted putting technique became known as "arm lock". This is where the grip rests (or locks) against the left forearm (for a right-handed golfer) during the stroke. The benefits are to help quiet the wrists and let you rotate the shoulders. The arm-lock style also creates a natural forward press or positions your hands ahead of the ball at impact.

First popularized by Matt Kuchar, other notable golfers who have successfully switched to an arm lock putting style are Bryson DeChambeau, Keegan Bradley, and Webb Simpson.  And yes, the arm lock putting technique does conform to the Rules of Golf.