I believe it will record (save) what's in the buffer, as mentioned, but you can stop it manually, of course after the 5-minute segment you want is over.It will record the entire program or the entire buffer, whichever is less?
I've always wanted a "trim" function just for this reason ! I've been watching something and an interesting segment is shown, so I do just like you mention, rewind to the start of the segment, press Record, then Stop when the segment is over. Later, I may tell my wife or one of the kids to watch what I recorded. They sit down and may have to watch 15, 20, 30+ minutes of what was in the buffer before it gets to what I meant to only record !