Force, leverage and strength in varying degrees depending on method used physically to break a neck. Force, leverage and circumstance in accidental or other situations.
A good friend is a pathologist Stateside. He developed a computer program to be used in trials to convict or exonerate the accused. He was tired of repeating himself many time a month in trials etc. He created a series of programs to detail all manner of deaths & injuries; knife, bullet, physical force. He has made millions off of this endeavor to save time. He also now has 3-5 employees working to keep the models up to date.