The term "Bang" in the title means a violent event (explosion, ambush, active shooter, etc.). To stay "Left of Bang" means that you see that something is about to happen and you already have or are actively taking the steps to counter the threat, a very proactive method to violent incidents.
Left of Bang was developed by the author for the Marine Corps' Combat Hunter Program. This will detail how to spot normal behavior, thus when someone is acting abnormally, their behaviors stick out like a beacon. This way you can focus on that person (or persons) and assess if they are a potential threat. Once you see a potential threat, you can work to stop the threat before the "bang" os stop it as quickly as possible after the "bang."
When I categorized this book in my library, I put it in the "Military" category by reflex, however it ended up under RKBA because I recommend this to help acquire situational awareness.
Mr. Van Horne breaks down in great detail the sub-conscious physiological traits we display when we are under normal conditions and under stress. If you can recognize how a person (for example) would get out of their vehicle, walk to and enter a store to purchase something (known as a "baseline"), then someone who is looking to shoot up / blow up / rob that store will stick out like a turd in a swimming pool. Your attention will be drawn to those person(s) because their behaviors are different. You can then assess if they are a threat and if so, take whatever measures you need to flee, alert others and/or engage (fight / shoot).
This is an excellent set of skills to possess, no matter if you are military, law enforcement and armed or unarmed citizen. This book is on my "re-read every 3-6 months" list to keep the knowledge and skills fresh.