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There are ass_shake posts like post #512667 and post #5141955 where the hips are swaying without any visible "bounce".
It doesn't help that bouncing_ass lacks a proper definition.
punished_K said:
active -> passive movement
If ass shake is for actively shaking one's own ass (i.e. twerking), and bouncing ass is for passive jiggling (usually during sex), then they're not really the same thing.
The bulk update request #10305 (forum #212533) has been rejected by @DanbooruBot.