One of the great things about a vertical golf swing is that it allows you to maintain equal pressure in each hand throughout the entirety of the swing. That's because your arms and hands go straight up and straight down...or at least they should!
It's really hard to keep the same amount of pressure in both hands when you're using a rotational swing. Because your hands can get so far behind you into what I call "the sacred burial ground", one of your hands is going to have to grab the club at some point to get it back to the impact position. This pulling motion is going to create an unequal feel in your hands, which can lead to all sorts of errant shots.
Remember, you need to have a firm grip with EQUAL pressure in both hands. Let them work together. They should be diametrically opposed, with one mirroring the exact movements of the other. Work on that vertical swing and you'll start hitting better shots much more consistently.
Keep it vertical!
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