It is a common aphorism in military training, related to the latency of effective fast-twitch reaction. The jerkiness of trying to point your weapon at a target as quickly as possible and then yanking on the trigger means you usually miss it.
The most important motion is bring your weapon to bear on target in a mechanically efficient way and pulling the trigger as it comes on target in a single motion. That single integrated motion can only be learned by doing it slowly but it is very accurate and smooth. If you practice the motion enough it becomes very fast. It is fine muscle memory. This is virtually always faster in terms of putting a bullet on target than relying on raw muscle speed. Also why military firearm skills are perishable, it has to be constantly practiced to keep the latency down.
Forgot where that's from but I think it's a Navy or Marine saying.