Tapes tend to stretch, especially after they’re used the first time. (I know perfectly well that modern tape vendors all claim that they prestretch their tapes, or that their tapes cannot be stretched, but that claim and two slices of bread will get you a bologna sandwich.) Retensioning a tape is simply running the tape completely through, both forwards and back, with the command mt retension.
What do you think he's trying to say there? Does this have something to do with the expression baloney which means nonsense talk?