I have the same brown mark across my thumb, 1/2" down from the tip.
Mine is caused from bending the thumb upon release, and hitting the front of the thumb hole.
I don't feelit, but have seen it in slo-mo video, at the release point.
No big deal, no pain...just discoloration.
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Arsenal:
13# Hammer Cherry Vibe (Undrilled)
13# Brunswick Red Alert Grande
13# Track Power Machine (Undrilled)
14# Track Inertia
14# Brunswick Power Groove Reactive (Undrilled)
14# Columbia Blue Dot
Duke Harding