Similar problem
I have struggled with a similar problem, and my sensei and sempai tell me that this, along with a tendency for the blade to descend through the final 1/3 or so of the cut (relative to speed. It's much more noticeable the more slowly you execute the cut.) with a side to side "wavy" type motion, is a result of two bad habits.
The first is gripping the tsuka too tightly and not having my hands/wrists placed properly, and the second is the use of too much power overall in the execution of the cut.
With practice, and really paying attention to my grip, plus use of a full length mirror to watch my whole waza when someone more experienced is not available to watch me have helped relieve most of the problem. I notice it still happens a lot when I start practice until I have really warmed up an loosened up my wrists.
Just my 2 cents. Hope it is of some help to you. Get your sensei to watch you execute slow cuts and stop you at the instant he sees where you are going wrong. That's how we diagnosed and began to correct my problems.
All the best!
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Chris Brown
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