The levers on 3port mid position valves are only there for filling purposes after draining down. It is used to turn the motor/quadrant/valve to mid position where it is held on a latch during filling ensureing water can enter both HW and CH pipe work.
Other than that it should be taken off the latch and left loose.
Sometimes it can be useful to test if the motor has in fact moved from HW. If motor has not moved there will be resistance.
Also with a dead motor it can sometimes be used to move motor/valve to mid position and get some form of heating and hot water until problem is fixed.
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