What fundamental technique involves focusing gaze slightly aside from a faint nebula to engage the most sensitive retinal cells?

Averted vision, which intentionally places the target in the periphery where rod cells are concentrated.

Averted vision is regarded as the most fundamental skill for successfully viewing faint celestial structures like nebulae visually. This technique requires the observer to deliberately avoid looking directly at the target object. Instead, the gaze is focused slightly adjacent to where the nebula is located. This maneuver strategically places the dim light onto the periphery of the retina. This area of the retina is densely packed with rod photoreceptor cells, which are vastly more sensitive to low light levels than the cones located in the center (the fovea). By maximizing photon capture in the rods, the eye is often able to register the presence of the faint object, sometimes causing it to suddenly 'pop' into visibility after a brief waiting period.