Police are allowed to use whatever force is necessary to arrest an individual or protect themselves or others if someone presents an immediate threat. The officer doesn't have to wait for the person to actually attack as long as there is suitable indication that the suspect does intend to commit harm or flee. Police must follow a "use of force continuum/matrix" that governs what type of force is allowed in the face of a given level of resistance. Typically, the officer can go one step above whatever resistance is being posed by the suspect. The steps for the officer typically are:
1) Officer presence
2) Verbal commands
3) Soft empty hand control (transport moves & pressure points)
4) Hard empty hand control (strikes, pepper spray)
5) Intermediate weapons (Baton, Taser)
6) Deadly force
Using the +1 rule, the officer can go one step beyond the suspect. For example, if the suspect is being verbally non-compliant (which is the equivalent to level 2 verbal commands) the officer can move to level 3 and lay hands on the suspect to gain compliance. The reason for the +1 rule is to allow the officer to gain compliance rather than trade equal force all night long. If the suspect doesn't comply, the officer can lay hands on them. If they become actively resistant, the officer can move to strikes or intermediate weapons, etc. The only exception is deadly force, which can only be used when the suspect presents the immediate threat of deadly force.