I believe he is referring mostly to positioning. You want DBs in a position to make a break on the ball and see what the QB is doing without getting beat. In essence, you want them to be proactive rather than reactive. This takes a great deal of coaching and, per the norm, speed. If a DB is playing catchup, he has lost leverage. The same holds true for LBs. In a zone defense, they need to get to their drop zones quickly. The quicker they get there, the earlier they can decide what to do. Then, they can play downhill. That is leverage. Speed, positioning, and the ability to play both the run and the pass.