What version of Super Training are you reading. I was looking to put the quote in context, but I have the 4th ed (not the most current).
If I had to guess based on your quote, it would have to do with muscle fiber recruitment. Top level strength athletes can recruit more muscle fibers due to their years of training and development. Everyone will recruit more under certain strain and levels of fatigue. This is a pretty simplistic answer, but I'd like to re-read the chapter/ context of the quote before going into more detail.
Siff and Verkhoshansky were both men of science who brought much to the sport of weightlifting. Supertraining is well worth the effort, but you need a reasonable foundation before it really clicks. Much of the writing and translations of Yessis are a good place to start for an overview of Russian/ Soviet training.