For years, TikTok has described its powerful For You Page as a personalized feed ranked by an algorithm that predicts your interests based on your behavior in the app.

But that’s not the full story, according to six current and former employees of TikTok and its parent company, ByteDance, and internal documents and communications reviewed by Forbes. These sources reveal that in addition to letting the algorithm decide what goes viral, staff at TikTok and ByteDance also secretly hand-pick specific videos and supercharge their distribution, using a practice known internally as “heating.”

“The heating feature refers to boosting videos into the For You feed through operation intervention to achieve a certain number of video views,” an internal TikTok document titled MINT Heating Playbook explains. “The total video views of heated videos accounts for a large portion of the daily total video views, around 1-2%, which can have a significant impact on overall core metrics.”

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Heating also reveals that, at least sometimes, videos on the For You page aren’t there because TikTok thinks you’ll like them; instead, they're there because TikTok wants a particular brand or creator to get more views. And without labels, like those used for ads and sponsored content, it’s impossible to tell which is which.

Employees have also abused heating privileges. Three sources told Forbes they were aware of instances where heating was used improperly by employees; one said that employees have been known to heat their own or their spouses’ accounts in violation of company policy. Documents reviewed by Forbes showed that employees have heated their own accounts, as well as accounts of people with whom they have personal relationships. According to one document, a heating incident of this type led to an account receiving more than three million views.

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Documentation about heating within TikTok and ByteDance is substantial, but poorly organized. Documents purporting to govern heating exist across multiple teams and regions, including the Content Programming and Content Editorial Team based in Los Angeles, and the Live Platform and Product Operational Teams, based in China. In addition to the MINT Heating Playbook, there are documents titled MINT Heating Operation Policy 101, Heating Quota Guidelines, TikTok Heating Policy and U.S. Heating Strategy Guidelines.

These documents suggest that TikTok and ByteDance initially turned to heating for a mundane, legitimate business purpose: to diversify TikTok’s content away from lip synching and dancing teens, and toward videos that would interest more users. “The purpose of this feature is to promote diverse content, push important information, and support creators,” says the MINT Heating Playbook. “If you make good use of it, heating resources will bring a leverage effect, a small amount of heating resources will bring about growth of midrange users, and a more diverse content pool.”

TL;DR: This also means that TikTok is picking winners and losers: creators and brands could lose a spot on someone’s For You page to someone that has a tighter relationship with the company. According to the post, there have been incidents where employees heated content they shouldn’t have, promoting videos from friends, partners, and even their own accounts.

This can also cause creators to lose interest in the platform if their videos underperform compared to ones that are being boosted, as TikTok’s lack of transparency around heating makes it hard to tell which videos got to the top organically or not, which then hurts the platform and creators and viewers.