This tag would tell a unit to not move forward at all when turning, optionally there could be a MinTurnSpeed and MaxTurnSpeed tag that define the lowest and highest speed a unit can move forwward at while turning.
The purpose would be to enable e.g. tanks to turn in place without moving forward while turning. This would also make limited arc units stay in their formation when aiming at something.
I think steering mode is supposed to do something similar, is that tag used by Spring?