This is an interesting article about the rules that govern police use of force.
Police departments reject the idea that use of force can be governed by hard and fast rules. Under this rule-resistant view, using rules to regulate use of force would be dangerous and in practice impossible, as officers must retain broad discretion to respond to ever-changing conditions in the field. Despite the prevalence of this view, the Article finds that, behind closed doors, departments are constructing hard and fast rules that limit officer discretion.
— Read on papers.ssrn.com/sol3/papers.cfm