I'm clearly being thick but I don't understand how he would lift 'one end' of the loop if he just said the two ends are joined together?
He's talking different "ends" here, the ends of the film and the ends of the loop. First, the two "literal" ends of the film are joined together, forming a loop. But once the process starts, you are holding the film with your two hands spread apart the length of the loop (so half the length of the full film, about 16-17 inches to fit in the 19-inch-long tank), forming two "ends" of the loop—right-side end and left-side end.
At least that's how I read it.
