What are police taught to do if they feel their lives are being threatened?

(CNN) – It is now clear that the account the police chief of St. Louis gave about the police shooting of a young African-American male on Tuesday – that he "pulled out a knife and came at the officers, gripping and holding it high" – is not true.

After CNN obtained cell phone video of the incident and notified the police about that fact, the police released the video to the public, which shows the young man was not holding the knife high at all.

There are other questions about that shooting of a man the neighborhood knew to be emotionally distressed, including why the two officers shot him nine times.

And the larger issue at play with this and the shooting of Michael Brown – just what are cops taught to do if they feel their lives are being threatened?

CNN's Jake Tapper reports.