This is a tool-assisted speedrun. For more information, see tasvideos.org/1692M.html
Capcom decided to make some easy money, so they recycled parts of Mega Man X5, threw in overly difficult trial-and-error gameplay, made almost everything an obstacle course, and called it Mega Man X6.
This run by Rolanmen1 and FractalFusion plays on the hardest difficulty, Xtreme, and uses both X and Zero. The physics of Zero's sword make it the ideal boss killer, however some bosses have unavoidable invincibility periods, so X is sometimes used to attack during these periods. Some new tricks like duck dashing and saber climbing help save 19 seconds over the previous movie.
If you want to see a run that plays through all of the stages, see this run: tasvideos.org/2342M.html