The fall of South Vietnam, April 1975 — Communist military movements and refugee flows, from the 1976 Britannica Book of the Year

Found this in a copy of the 1976 Encyclopædia Britannica Book of the Year that I've had since I was about 11 or 12. I grew up in Iraq and later Canada and had no personal connection to the war whatsoever — but this map stopped me cold every single time I opened the book. What makes it work so well: