Light-piping
From Transformers Wiki
Light-piping is a low-tech gimmick used on Transformers toys that allows lighting effects (usually for the eyes) without the need for electronics. Light-piping is achieved by using a translucent (often colored) piece of plastic that is inserted into the body part in question (usually the head), leading from one side of the body to the other side. When the other side (usually the rear, sometimes the top or the sides of the head) is held facing an external light source, the light will shine through the clear plastic and cause a lighting effect on the front side. The gimmick was originally introduced with the European-exclusive late-G1 Turbomasters and Predators toys released in 1992.
Some more advanced light-piping techniques allow for less conspicuous windows that don't always even have to be on the back of the head. For example, both Energon Shockblast and Alternators Shockblast feature a triangular design facing forwards that manages to pipe light through to their singular eye, meaning they don't have to be held with their backs to a light source.
Being a fairly widespread feature, there are also numerous instances where a toy was initially designed to include light-piping only to have it "neutered" at a later stage — usually by employing opaque plastics instead of translucent, or covering one or both ends of the "pipe" with paint.
When used outside of eyes, light-piping is most often used to allow weaponry to light up without storing electronics within the weapon itself; that is, by having a "pipe" starting in the weapon's handle, and embedding a light in the weapon's owner's hand, so that when the weapon is held, lighting the hand-light causes the weapon to light up as well. For example, Armada Optimus Prime and Masterpiece Grimlock use this trick.
Light-piping has waxed and waned over the years, with some lines featuring it in most of their figures and others neglecting it entirely. In general, it tends to be more common when the figure already has translucent plastic incorporated somewhere else (most commonly windows or cockpits), which means not having to cast a new mold for a single small piece. Many instances of "neutered" lightpiping occur due to the eyes being cast in an unfitting color - for instance, War for Cybertron Soundwave's pipe is blue, since it shares plastic with his chest window, and was therefore painted in red.
