In the last lesson I included an animation of what I consider to be a perfect exercise to simulate the pulling motion and the slot. In that animation, the instructor puts his palm flat against the wall behind him. As he starts to open his shoulders, he begins to drag the palm of his hand forward along the wall. This simulates the pulling motion of the stroke as well as the biomechanics Gary Adelman describes in his quote.
A second, equally perfect exercise comes from Nick Bollitieri's brilliant Killer Forehand instructional video. I had never heard of the modern forehand as being a pulling motion before watching this video, and this is what opened it all up for me. The video has a guy pretending to be a goofy scientist for entertainment, but don't let that fool you. This is brilliant. Here is an excerpt from the video showing the "pull the towel" exercise. Xavier Malisse is demonstrating the exercise.