In this case I'm going to agree with Adema (yes- I'm agreeing!)
The zipper line as we always referred to it when judging on the pro mogul tour was as Adema described, skiing the "seam" (I like that term) between two sets of vertical bumps. To envisage a zipper just coming together or apart, that "seam" represented both the falline, and the path skied.
It was nothing more than an over-skied, grooved line which dominated/ dictated the path of the skier. And indeed, was the fastest way down the hill. (nearly straight)
Still fun to do , but there are certainly more civilized ways to ski bumps...