The term used when you call someones name from behind or tell them to look somewhere, causing them to suddenly turn their neck in order to look. Hence, they move their neck in a violent manner as if to figurativly "break their neck".
Little Tim is walking and Stuy yells,"Hey Timmy". Little Tim turns suddenly and Stuy hollas "BROKE NECK!".
When a fly honey walks by and a guy or group of guys suddenly jerks their head to look. Like when you pass some fine-behind chick and your body stays forward but you twist your neck to get veiw from the rear. Your neck isn't literally broken unless you seriously jerk too fast. That happened to a friend of mine.
As a hot babe walked by, Ricky and Chris took a double-take both got a broke neck.