Specifically, pornhub. Just above the timeline along the bottom of the frame there is a grey silhouette, like a mountain range. What are the peaks and valleys supposed to be? (For reasons I don't need to go into, I almost never have the sound above a whisper but when I do listen carefully, I don't think the silhouette is a volume indicator.)  
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What COULD it be? A hotness indicator? Number of times those sections were rerun or paused?  
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I noticed it when I clicked on a clip with Empera. Cute and sexy. Wow.