Dragon Ball Super introduced an important element into the "Universe Survival" saga's Tournament of Power, as Goku manifested a new power called Ultra Instinct, during his epic battle with Jiren of Universe 11.
The distinguishing characteristic of Ultra Instinct is that it allows a fighter to react faster than the speed of thought - i.e., instinct and fight technique that happens on an unconscious level. It's a power that the Gods of Destruction know well, and one that even Beerus has struggled to master.
Given the massive power that Jiren has already shown, and the fact that was only able to barely phase him in battle, even with an Ultra Instinct power-up, many fans are speculating that Jiren is also using Ultra Instinct, possibly in its fully mastered form.
The main theory about why Jiren is using Ultra Instinct (via Otaku Art) is simple: since the Universe 11 fighter is able to keep up with Goku's Ultra Instinct power levels and fight technique, it stands to reason that Jiren also tapping into that power.
Jiren is described as being as (if not more) powerful than a God of Destruction, and since Ultra Instinct is something the Gods must work at to manifest, it's conceivable that Jiren has mastered it.
The other evidence is the nature of the power: While Goku went through a pronounced transformation to manifest Ultra Instinct, gods like Whis or Beerus do not. So, the fact that Jiren doesn't manifest the same kind of transformation to keep up with Goku's Ultra Instinct state is irrelevant; if the Universe 11 fighter has truly mastered the power, there would be no visual clues other than the crazy levels of speed and power he's already demonstrated.
While there's a lot of evidence to suggest that Jiren is tapped into a power like Ultra Instinct, there's just as much evidence to suggest that he is not.
First of all: while gods may not show visible signs of tapping into the Ultra Instinct power, so far, our only real evidence is that mortals (like Goku) need a transformation to achieve it. That may change fundamentally as Goku masters Ultra Instinct, but for now, what we've seen is all the evidence we have.
Even more so than the visual evidence, are the implications of the character reactions to Goku's transformation. When the Saiyan first taps into Ultra Instinct, the gods watching the tournament (Beerus, Whis, Belmond) are clearly shocked, and comment about how surprising it is to see a mortal access that power. And yet, the gods have never made mention of Jiren using the power, or how he and Goku are battling each other on that level. That's a pretty big clue that whatever fight technique Jiren is using, it's not Ultra Instinct.
One theory is that Jiren is actually telepathic. The Universe 11's appearance (like a stereotypical 'little green man' alien) could hint at the equally stereotypical psychic abilities that are often attributed to such UFO theories. Jiren being psychic would also explain why no the Tournament of Power fighters can barely touch Jiren; why Goku is finally able to land some attacks in Ultra Instinct state (no telegraphing thoughts to read); and why even Hit's surprise attack fails.
In the end, this will all be fan speculation until Dragon Ball Super provides the answers. What do you think: Is Jiren a master of Ultra Instinct, or does his impressive power stem from a different source?
Dragon Ball Super airs on Crunchyroll Saturdays at 7:15 p.m. CST. Toonami also airs the English dub on Adult Swim Saturdays at 11:30 p.m.