I used to watch the guys on the assembly line install windshields. Two guys worked together. One guy was half in and half out of the car as he reached in and grabbed both ends of the rope. He jerked the rope in a rhythmic tempo while both slapped vibrations on opposite sides of the glass from the outside.
The rope came vulcanized into the weatherstrip so it didn't come out very easy and no lubrication was used. You can imagine, as the rope was jerked, it became long in the guy's hand. That's why he jerked it out in 'bites'. Then, I noticed a box full of spent ropes.
At a rate of 1,000 windshields per day (one per minute), they sent the used ropes back to be re-vulcanized in new weatherstrips. Each rope had splotches of black on the surface; evidence of little pieces of the many weatherstrips from whence they came.
If memory serves me, I believe the ropes started at the top. Of course all their actions were meticulously orchestrated but it seemed to me that their slapping actions were the most important in seating the glass in the weatherstrip and in the aperture of the car body. These weren't little love taps and every once in a great while a windshield would crack, especially when they weren't in unison. - Dave