30/60/90.
30% pace, 60% pace, 90% pace.
In a nutshell, the exercise works like this- baseline player hits a drop shot at 30% pace to net player's feet. Baseline player follows that shot with a 60% pace ball to the feet of net player who has dropped back one or two steps. This simulates a more realistic situation in which an approaching player isn't yet at the NVZ line and the baseline player wants to keep the ball low to the opponent's feet. Baseline player then hits a flat drive at 90% pace, trying to jam the opponent who has stepped up to the NVZ line again. Repeat the series.
NVZ player ought to handle various speeds and attempt to return deep to baseline to challenge the baseline player.
To make the drill more difficult, NVZ player can alternate between left and right side of the court while maintaining the rally.
Both players get challenged a lot during this drill/exercise. You'll be gassed after a short period of time due to the high intensity. Make sure to switch roles.
This is outstanding practice for both players!