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Don't they have the option to power down only a little bit of steem power they have? Do they have to power down ALL of it?

They can toggle it on and off. Any time it is on it will be AS FAST AS THEY CAN. So your screenshot is a sample that suggests they are collectively powering down at about 60% speed.

No, no option. So yes, all of it. But they can stop that process whenever they want.