Directed by Star Wars/Alien art director Roger Christian, Black Angel was a sword-and-sorcery short film famous for pioneering the use of "step-printing" to slow down the action in the fight scenes. George Lucas was apparently so impressed with this technique that he decided to use it in Empire Strikes Back.

According to Ars Technica, the real reason for slowing down the fight scenes wasn't to make them look cool but to make the short film long enough:
There wasn’t enough initial material for Black Angel to fill the required 25-minute runtime. “So we slowed down fight scenes in it to build up the time,” Christian said. They did this through a technique called step-printing. Strategic scenes are shot at a slower film speed so action is sped up, then frames are printed at a slower speed onto the finished film. Action becomes very fluid, with individual moments seemingly smearing together with the next.

“I’m just happy Alan showed me how to step print in those days. I loved the look; it just transformed into some mythic areas,” he said. “But literally that was because we didn’t have enough film to up the time to the 25 minutes it had to be. All this was running through my head during the first screening, but Lucas absolutely loved it. So did other people.”
Learn more about this and other classic filming techniques by reading the full post here