Dynamic deviation control means not holding the gun totally still, but holding it still enough and pressing the trigger as smoothly and rapidly as possible to get the hit you need.
Holding the Gun Still
Many traditional target shooting and marksmanship principles are designed to help you hold the gun as still as possible throughout your trigger press, or maybe even help you stage your trigger so you shorten a trigger press, by doing things like isometric tension (pushing and pulling with opposite hands), controlling your breathing, and having an upright posture so your weight is on your skeletal system instead of being held up by your muscles.
These principles help you shoot tighter groups or hit more precise targets farther away. They are not defensive-shooting skill-based techniques. They are for controlled environments for extreme marksmanship skill development. Many people utilize these techniques in their defensive shooting drills with the idea that these marksmanship-type skills somehow apply to or will enhance defensive shooting. Rob Pincus believes this is a major misconception. Defensive handgun training and practice should not look like marksmanship practice.
Dynamic Deviation Control
Defensive shooting is in many ways different from target or marksmanship shooting: the defensive shooter will not be controlling his breathing, will not have an upright posture, will not be able to focus on holding the gun as still as possible, so why practice these things in defensive shooting drills?
Rob utilizes dynamic deviation control, which he believes is applicable to defensive shooting. It accepts and even embraces the facts that the gun is never going to be perfectly still when you are shooting, and that the shooter in a defensive encounter does not respond like a machine and is not shooting at a stationary target—the bad guy will probably be moving.