The strategy is called counter-illumination. Ventral photophores emit light directed downward, and reflective and pigmented structures shape the beam so that the belly's brightness approximates the background downwelling light. A predator looking up sees no dark gap where the fish should be.
Matching the background is not a fixed setting. Light in the mesopelagic zone changes with depth, time of day and cloud, so the output has to be adjusted. Experimental work on counter-illuminating fishes and squid has shown that animals modulate emitted intensity in response to changes in overhead light, and hatchetfishes possess upward-looking tubular eyes suited to sampling it.
The hatchetfish's silhouette is itself an adaptation to being viewed edge-on. Its extreme lateral compression leaves very little cross-section from below, and its silvery flanks reflect ambient light sideways, which reduces the fish to almost nothing from most viewing angles.