This is either a work of art or a work of madness. Apparently, via heavy use of speedrun tools that allow a person to frame-by-frame step through an emulated version of a console game, someone’s built a speedrun that uses one controller input on Megaman 3, 4, 5 and 6 simultaneously.
Advice: play this on Fullscreen, with the volume down. Chiptune music is grating enough to some folks that having four simultaneous tracks playing might just be too much to handle.
Tool-assisted speedruns are fascinating in that they often do some things that a human player could never pull off. This is probably only possible via manipulation of the synchronization in subtle ways — passing input to one game while another is in a screen transition or on the “weapon get” screen, or having one game walk into a wall while another is proceeding through the level, for instance.
This run found its genesis in someone joking that Capcom had basically produced the same game for every Megaman sequel. While they are certainly formulaic, the challenges all build off one another, and the Megaman games are traditionally grossly unforgiving which makes the speedrun all the more impressive.