When I was in Army night vision training, they told us that using peripheral
vision works better for two reasons:
1) The image is not as clearly focused and therefore affects more rods.
2) The high concentration of cones at the focal point means the
number of rods is smaller and therefore the ability to detect contrast at
low light levels is small.
3) When using peripheral vision, you don't focus on exactly the
same point very long, so visual purple has time to become replenished as
your eye roams. In fact, we were told to look around places where we
expected enemy movement in a clock or star pattern to use avoid exhausting
visual purple in one place.