The LiveTV ring buffer
A ring buffer is a chunk of memory or disk space that's treated like it's a ring: it has no beginning or end, MythTV starts recording live TV at the beginning, and when it gets to the end, it merely keeps going again at the beginning, keeping track of where the end is.
The playback side of the house checks that end, and never goes beyond it.
The depth of the ring buffer (the amount of total configured space) determines how far you can back up in LiveTV mode, and how long you can pause. If you leave LiveTV paused long enough to fill the ring buffer, Myth will unpause the video automaticly so that it it doesn't have to start writing over unwatched video.