Multitasking.

The guy covering down the hallway (black vest) maintains physical contact with the guy opening the door (blue vest), so that he can feel when his partner opens the door and rolls in...

Standing flat-footed and shooting a target is not really a super-complicated task once you get the hang of it. Sure, being extremely precise can demand a great deal of focus, but just plunking an A-zone or a bowling pin at twenty-one feet is not like performing rocket surgery.

In the action pistol sports, there's a joke that goes "Instant Idiot: Just add timer." Similarly, on the tactical side of things, Pat Rogers likes to say that putting a loaded gun in somebody's hand robs them of ten IQ points.

Part of the reason for practicing shooting and gun handling so that it reaches a certain level of automaticity is that running a stage or going through a shoot house (or having to use your gat for reals) is not just a shooting exercise, it is problem-solving with a gun in your hand. The more conscious mental "processor cycles" you have to put into running the gun, the fewer processor cycles you have to deal with not only the unknown or unexpected, but simply routine tasks like walking from Point A to Point B without setting yourself or anybody else on fire.

...and can then roll in behind him. He needs his brain power for watching which way his partner goes, as well as processing what's in front of his eyes.

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