As Knight, hold down m1 to attack as normally, and then hold 1 to do the Charge animation. Do not release 1 after pressing it, as this will not work. Keeping m1 held down, release the 1 button to charge, and if it is done right (the Knight needs to move its arm back to "wind up" the charge - this does nothing in normal play, it is just an animation thing), the Knight will be attacking at very high speeds. This is not a visual glitch.
A good way to see the effect this has is to go to the Sky world, get a bandolier with martial and shrike emblems, turn on combat numbers, and fight a Radiant Giant. On my Knight that has around 7k damage, after activating flask effects and performing the glitch, I can get 1/4 of the giant's health down before the flask effect wears off.
The glitch effect typically goes away when the Knight gets hit. However, if the glitch persists, the Knight's Retribution proc can hit multiple times.
Another glitch can be found using the flask arrangement given above, where for some reason, enemies attack far less frequently, if not at all, when the flask effects are active.