The most simple version of the answer I know is that below a certain MP, there is not enough power to maintain the high rpm set by the prop. If the prop control doesn't have an effect, you are essentially sitting there with a fixed pitch prop, so it reacts to changes in MP just like a fixed pitch prop. It will do it in the air just like on the ground.
I'll stick with ananoman's answer to the rise in MP during the prop cycling. Can't get any clearer than that.
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