I interviewed a police training specialist this past week, and asked him why he orders his officers not to put their finger on the trigger when they draw their guns. I thought his response, by e-mail, was interesting:

We train officers to put their finger on the trigger only when they want a bullet to leave the barrel. There is a natural desire to put the finger on the trigger when one anticipates needing to fire quickly (such as covering a dangerous suspect with his hands hidden from view). However putting the finger on the trigger prematurely increases the risk of an unintentional discharge. The amount of time saved by having the finger already on the trigger is only about 1/3rd of a second. That time savings has to be weighed against the known physiological effects of high stress arousal, such as a loss in finger dexterity and muscles tensing. The fraction of a second lost can be made up through better tactics like using distance and cover to improve an officers ability to react to a threat. ...

On two occasions I have had to suddenly react to a deadly force threat. In both instances I decided to fire and reacted instinctively. owever, the "slow motion time effect" I experienced enabled me to continually observe the threat. In both cases I was able to hold my fire as the situation changed in the 1-1.5 seconds that elapsed before the trigger was fully pressed. One of those situations included an unarmed man jumping in the line of my intended fire.