I have a bunch of DirecTV Tivo's and if you record a show in the middle of the show, it'll record everything in the buffer along with whatever is coming up. So if you were watching letterman the whole time and didn't change channels and then hit record, it saved your buffer (up to 30min).
Now if you had changed channels right before Tom came on and then switched back to Leno, the buffer would be gone and it would only have recorded from that point on.
I don't know if Stand alone tivo's work the same or not..