We operate with two kinds of memory. Automatic or Implicit memory is where we recall things without thinking about it. Things like putting one foot in front of another to walk, riding a bike, etc. Recalled or explicit memory is where you recall information at will, like remembering a phone number or address, multiplication tables, the names of acquaintances, etc.
Training drills are intended to build the neuromuscular memory to imprint it as automatic memory that requires no conscious thought to perform a physical task. Here I point out that "muscle memory" is a misnomer, because the memory resides in the brain, not in the muscle fibers. Fundamentals like grip, the draw, safety and trigger management, presentation of the gun, and sight alignment are skills we want to build into automatic memory so we don't have to take time to think about them under pressure. As an example, I and fellow officers shared the experience of having the gun in hand when a threat appeared, but no awareness of having drawn the weapon. The goal is to build those skills so firmly into automatic memory that they are instinctive. That's the foundation we build upon.
We know that cognition, and explicit memory diminishes at times of extreme stress. Our ability to recall information that facilitates survival in combat relies upon education and experience. The new guy in a combat unit, or a new cop on the street, is more vulnerable until he has the experience to function under high stress. Realistic training scenarios can help but individual brain functon is crucial.
Now consider the disadvantage to an individual who has not mastered the firearm handling and shooting fundamentals in automatic memory, who is thrust into mortal conflict. This is where fundamental skills imprinting is critical.