When does one unzip the window?

I have a 1965 Plymouth Valiant Signet with a manually operated convertible top. Never really had any training on how to lower/raise it. My question concerns the plastic zippered rear window, is it supposed to be unzipped before lowering the top? I just leave it in place, it's a little hard to push the top down, but it seems to work. Don't know what the zipper is for in the first place.