Currently we have a sensitivity slow down or "turn cap" while releasing a swing.
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The pros of this turn cap are obviously to reduce the possibility of actually spinning 180 degrees while releasing your swing, which I fully agree with the intention of mitigating.
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The turn cap was the thing that got applied in the first patch as soon as you started charging a hit, and now got changed to be permanent, as soon as you spin more than 360 degrees.
However, the slow down after releasing the swing has always been there, and it's reason is NOT to prevent spins.
In MO1 you could change direction while releasing, and people with spears abused it heavily to drag their spear once across their whole screen to get a hit, without actually needing to "thurst" at a certain position.
It's there to "blanace". Overhead strike hits only a straight line down. You're not supposed to use it on enemys that are in full movement (you could try, but it's hard to hit). If you allow free movement then you basically make a normal slash out of the overheadstrike.
And it's also there to make positioning more important. In MO2 handle hits of clubs, without proper position, reduce your DPS heavily. To get good DPS out of a mace you need to position in a proper distance to the enemy, and aim decently before releasing.
The time after the release is only supposed for small adjustments, and not to reposition the whole swing.