<or does the twine get pulled out from
under the rubber thereby zipping the rubber lip over the metal?>

Thats it. As you pull on the cord it draws the rubber lip over the edge. I've done countless Bays, and one Vanagon. The Vanagon was actually a little harder than the Bays, which surprised me since the Vanagon windshield fairly flat compared to the Bay. The worst one yet: a month ago, replacing the winsdhield in my '74 Super Beetle convertible-had to start over 3 times as it kept popping out. Much sweating and many nasty words!    Jeff