Here's the secret sauce: "Using machine learning, the algorithm serves videos to users based on their proximity to other clusters of users and content that they like." It's impressive how they arranged to be transparent for everything else, but kept this important part opaque.

Rumor is that the actual secret sauce here is human curation; people hand-select videos with high appeal and label them "viral", "popular", etc. in order to astroturf eyeballs and clicks. I suppose that admitting this directly would contrast sharply with the Chinese-harmonious-technocracy veneer that they work to project.