SOPs on such things vary widely from agency to agency.
Something along these lines is probably the most common interpretation, though:
Electro-muscular incapacitation devices are appropriately deployed when a reasonable officer perceives the legal and tactical grounds for initiating control of a subject and determines that noncompliance on the part of the subject exists or is likely.
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It is typical for agencies to record significant reductions in injuries sustained to subjects and officers following the adoption of EMIs. Simply put, it stops many physical confrontations before they escalate, and in many instances, before they even begin.
