You should be able to see the back of the intake valve through those holes. When the intake valve opens, air from the rocker cover flows through those holes past the intake valve into the cylinder.

At high throttle settings, the air in the intake tube and rocker cover is at a pressure higher than the air outside because it's been "pumped up" a bit already during the previous power stroke by the downgoing piston and sealed off to hold that pressure with a disk valve.

I wonder if "stuffing" the rocker cover would increase the supercharging effect?