So nice to see that I am not the only one who loves Columbo :)
Well, the episode I mean takes place at a cinema, not at home. The woman is actually in charge of changing tapes. The change is seen at the screen as a white spot, and the position of this spot (its not there, its too late, too soon or something)is what Columbo uses to frame the murderer.
It has something to do with the time she uses to kill the guy, its only a couple of minutes, then she is back at the controllingroom.