When editing videos without re-encoding, your "A point" (beginning of the segment) does not have to be on an I-frame. Only the "B point" has to be on an I-frame.I was cutting commercials out of some TV shows I have recorded. the problem is the I-frames don't always come until after the commercial has started. So if you go to the first I-frame in the commercial then step back a frame at a time you can make sure to cut any of the commercial that was in the P or B-frames from the last I-frame. You can not do the same on the "B point" end since you would be leaving P or B frames without an I-frame to reference.