When you throw a slider, you don't pronate at all. In fact, it's the opposite of pronation. A slider you are "cutting" the ball, slightly turning the wrist inward. Pronation is turning the wrist outward. Pronation also has nothing to do with pressure on the ball. When throwing a changeup, I've never heard of someone using different finger pressures to get specific spin.
When throwing a changeup, make sure the grip is loose. When I throw it, the pronation aspect is forced by me telling myself "throw the thumb at the catcher".
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