Jim Furyk debuted an upside-down putter grip (the rubber grip, that is) at the Transitions Championship -- his first win since mid-2007. When asked why the upside-down grip helped, he said he didn't know, although he wondered if it had something to do with his crosshand grip.
It didn't. It can help anybody with any kind of grip; it just depends on what you're trying to accomplish with your putting technique. Today I'm going to tell you how it works, so you can determine whether it will help you. The illustrations I'll use are of a righthander's reverse-overlap grip, but the concept is so simple that you should immediately be able to apply it to any grip, whether you're right- or lefthanded.
In my book I published a list of seven principles all good putters from several eras seem to agree on, and I also posted them here. You'll notice that Principle #4 is "The putter handle should be held so that the shaft aligns with the forearms." That's the principle that Jim's upside-down grip affects.
Here's the standard grip:
When you flip the grip upside-down and take your normal grip, the extra rubber under the shaft causes it to angle upward. It's no longer aligned with your forearms:
To get the shaft back in line, you have to arch your wrists upward, which means your hands tilt down:
Here's one more picture; showing the arched wrist with an open hand may make the positioning a little clearer:
Now your most likely question is "What's so great about arching your wrists this way?" The answer is, it depends on what you're trying to do with your putting stroke.
Installing your putter grip upside-down can help if you're trying to minimize wrist movement during your stroke. There are two main ways your wrists can flex during your putting stroke -- up and down, or back and forth. What you may not realize is that the combination of the two can result in a flipping motion, where the toe of the club flips past the heel through the hitting area. The results of this are unpredictable from one stroke to the next, and has the same result as twisting your forearms during the stroke, which is the number one problem for most players.
Arching your wrists not only eliminates some possible wrist movement, but also moves the weight of the putter shaft more under your forearms. With the weight under the pivot point, it makes your wrist position even more stable, further minimizing any possible rotation. It does this no matter what kind of grip technique you use.
Will this help you? That depends. I like to use a putting technique where my wrists flex more than is normally taught today, therefore it would likely interfere with my motion. If you follow some of the modern teachers like Dave Pelz or Stan Utley -- teachers who prefer that you swing more with your shoulders -- then an upside-down putter grip could help you putt more consistently.