The LiveTV Ring Buffer
Pre 0.19 MythTV used what is known as a Ring Buffer for handling LiveTV.
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 automatically so that it it doesn't have to start writing over unwatched video.
Myth verions 0.19 and beyond have changed the way they handle LiveTV. All recordings are now treated the same way. LiveTV recordings are kept in a separate recording group called LiveTV. They will be expired before any other recordings. They can also be expired automatically after a user configurable number of days. The change allows users to watch a LiveTV recording and then keep it by changing the group from Live TV to another group. This prevents Myth from expiring the program on the LiveTV schedule.