Kuhnhausen says: "The primary difference between the direct gas impingement operating system used in the M1 rifle and the gas cutoff and expansion system used in the M14 rifle is that initial rearward movement of the (separate) M14 gas piston further meters propellant gas flow to the gas cylinder by blocking the gas port and, literally, cutting off the flow."
Shortening the op-rod from M1 length required tapping the barrel at a point earlier in the operation cycle (bullet closer to the chamber) and where pressures are presumably higher as well. There's way more energy available there than necessary to cycle the op-rod/bolt, so metering was necessary.
It says something of the pressures in a barrel that the M1 works as it does. There's about 1.5" from the gas port to the muzzle for the gas to do it's work on the piston before the bullet exits and barrel pressure vents. At a bullet velocity of 2700 fps, that's 45 microseconds, timewise.
