The tuners are always active and recording into its own buffer, so that even if you aren't actively saving a recording, you can hop back and forth between different tuners and have full "trick play" functionality (rewind, skip back, etc).
Some people who like watching live tv alot will sometimes use two tuners and hop back and forth, say for example two different live sports programs. (The 'last' button can come in handy for that exact thing)
Another thing is if you are watching a program "live" and decide to hit 'record', it will actually save as much of the show as it can, as much has been captured in the buffer and then continue recording through the end of the program.
The tuners remain on the last channel they were changed to, whether it was by a human, to record a scheduled recording/wishlist, or to record a suggestion (if enabled) until tasked to do otherwise.
-Roamio Pro/Slide Pro
-Comcast/TiVo Mutant dvrs - RIP
-2x Upgraded Series 2 - Retired