forcescience's articles are in bitmapped PDF so i can't clip them, but i can google them:
Immediate vs. Imminent Threat
Attorneys and activists often assert that cops shoot first and ask questions later. Dr. Lewinski demonstrated that officers must take preemptive action in order to adequately defend their lives. If they wait until they actually see a suspect's gun pointing at them, it's too late. Exhibiting outtakes from scores of studies he has done on action vs. reaction and the lag time between the two, Dr. Lewinski drew sobered responses even from veteran firearms trainers in the crowd. Time- coded video of a slightly built woman who had never before handled a handgun revealed that she could draw from her waistband and fire faster than the average officer could react from a wide variety of 'ready' positions. Only .07 seconds elapsed from the time her gun was visible until she shot. Reacting officers were not able to beat her when they had to draw (average time 1.5 seconds) or even when their weapons were out in a low- ready position, a close-ready, a belt tuck, a 'Hollywood high guard,' a behind-the- leg 'bootleg' position or 'freed up' in an unsnapped holster. Indeed, trained officers are even slower responding from a bootleg position or with a holster unsnapped than in drawing a holstered weapon. In some positions, the lag time was mere fractions of a second.
But if you think this is a mouse turd in the real world, you're wrong, Dr. Lewinski stated. From most positions, especially if an officer has to visually confirm a threat, you'll have a round coming at you before you can react. Just one second equals four rounds from a Glock. Still, he recently encountered an opposing expert witness, a lieutenant from a major department, who swore in court that his officers were held to the standard of not shooting until they see a gun pointed at them. In other words, they must face a clear, present and immediate deadly threat or shooting is not justified by department policy. More realistic is what Dr. Lewinski calls the imminent threat standard. This involves an officer's reasonable belief that a potential threat is beginning to unfold that may culminate in his being placed in lethal jeopardy. This would include shooting on the basis of furtive movements. Laws in most states accept the imminent threat standard, Lewinski said. But as evidenced by the lieutenant's testimony, some departments choose (unrealistically) to set their standards higher than state law.