This video has the best example of butt clenching that I've come across. I absolutely love the way her red ass turns white and scrunchy. Growing up I once witnessed my older cousin getting spanked (origin of my spanking fetish). I remember her butt was clenching just like that, except she held it clenched like that the whole time. I'd love to see more videos like this for sure, but oddly I've only ever seen this one.
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Here's one from Moonglow featuring two girls spanked, strapped and caned by employer. The one to watch is the blonde, Elena or Helena, I think. Not so obvious on the spanking sequences although she does clench but very much so in the strapping/caning ones. She often clenches her bottom an instant before the impact.
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