Explanation
Rewatches count, and they multiply total watch time without adding a second of length. A twelve-second video watched twice generates more time than a twenty-second video watched once, and it does so while keeping the completion rate that shorter videos earn naturally.
The construction is straightforward and underused: end on the same framing, the same line, or the unresolved half of the question the video opened with. What breaks loops most often is a call to action stapled onto the end.
The common mistake
Assuming a loop needs to be a trick. The subtle version, where the last sentence flows into the first, works better than the visual gag version because it does not announce itself.