Thank you so much! I always wondered about that. The relevant bit, since it's near the bottom:
> The final step was to bring it all together with Periodic Intra Refresh. Periodic Intra Refresh completely eliminates the concept of keyframes: instead of periodic keyframes, a column of intra blocks moves across the video from one side to the other, “refreshing” the image. In effect, instead of a big keyframe, the keyframe is “spread” over many frames. The video is still seekable: a special header, called the SEI Recovery Point, tells the decoder to “start here, decode X frames, and then start displaying the video”–this hides the “refresh” effect from the user while the frame loads. Motion vectors are restricted so that blocks on one side of the refresh column don’t reference blocks on the other side, effectively creating a demarcation line in each frame.
> Immediately the previous steps become relevant. Without keyframes, it’s feasible to make every frame capped to the same size. With each frame split into packet-sized slices and the image constantly being refreshed by the magic intra refresh column, packet loss resilience skyrocketed, with a videoconference being “watchable” at losses as absurd as 25%.
