Expanded magwell for Tokyo Marui and WE Hi-Capa 5.1 / 4.3 series gas blowback pistols.
Plain and simple design, lacks the flash of some fancier designs but sturdy enough to take knocks and abrasion like the best of them. The expanded magwell serves to guide magazines to facilitate the quick loading of magazines.
These days though, they have also become a staple feature for race-style guns as much as superfluous but technically functional spoilers on urban sports cars. They do work and even make a difference but the fact that they look cool does not hurt.