I’m not sure of the Instagram algorithm, but I have observed that the TikTok algo does the same thing; it’s basically testing the video against a small audience first. If the video has good engagement/view time with that small audience, then it will show the video to more people. If the video performs with the wider audience, then the algo will show it to more and more groups of people until performance levels off when compared to competing videos. That’s how videos go viral.

If the video does not do well in the initial test, then it essentially withers.