Cards

Cards appear as a teaser at a moment you choose, then collapse into an icon in the top corner of the player. You get up to 5 per video, pointing at another video, a playlist, a channel, or an external link if the channel is in the Partner Program.

Placement is the whole job. A card asks somebody to leave mid-video, which is exactly when they are least willing to go, so the moment to use is the one where the video has just created the need.