After a string of critical and box-office misfires, Zack Snyder is basically done with the DCEU, aside from serving as an excutive producer on its movies. That being said, the filmmaker had some big plans for this cinematic universe that, aside from Wonder Woman, has never had a major critical hit. After Snyder's abrupt exit from Justice League's production last year, we'll likely never see his fully realized vision for Superman, Batman, and the rest.
Nevertheless, that doesn't mean he can't tell us what he had in mind when he was at the top of his powers at Warner Bros.; or maybe it does mean that, because, you know, contracts and NDAs and all that. Anyway, while interacting with a fan on Vero, the director's social media app of choice, Snyder revealed that Barry Allen/The Flash (Ezra Miller) was using the cosmic treadmill in Batman v Superman: Dawn of Justice.
“Even [though] Bruce knows it’s dangerous to occupy the old Bat Cave, it has to do with time travel from one point to another. i.e. Flash travels from Cave in the future to Cave in the past, but he is ‘too soon,’ meaning he went too far back, also meaning he will have to step off the ‘cosmic treadmill’ one more time," Snyder wrote.
The cosmic treadmill is a time-travel device that's been in the DC Universe since the early 1960s. It was built by Barry Allen, who, using his own super speed, could transport himself to the future or past simply by running on the tricked-out exercise machine. Prior to the Crisis on Infinite Earths, the Scarlet Speedster was also able to use the treadmill to travel to different dimensions and realities.
The aforementioned scene in Batman v Superman finds Bruce Wayne (Ben Affleck) nodding off in the Bat Cave when he has his infamous Knightmare about a dystopian future ruled by Superman (Henry Cavill). Then the Scarlet Speedster shows up via some sort of shining wormhole to deliver a warning:
"Bruce! Listen to me now! It's Lois! It's Lois Lane! She's the key! Am I too soon? I'm too soon! You're right about him. You've always been right about him. Fear him. Find us, Bruce! You have to find us!"
Don't you just love the pronoun game? The "him" is seemingly Superman, and the "us" is the members of the Justice League. The fact that Lois (Amy Adams) is "the key" led some to believe that if she were to die (a la the Injustice storyline), Superman would become the insane warlord that Bruce saw in his dream. While this cameo was not referenced in any subsequent DC features, Snyder has at least confirmed the way in which Allen was able to travel through time.
It is possible that this sequence could tie into the upcoming Flashpoint movie, where Allen finds himself in a future devoid of superheroes. Directed by John Francis Daley and Jonathan Goldstein, the standalone project is scheduled for release in 2020. Once he reunites Earth's heroes, he might find it prudent to go back in time and ensure that Bruce not only assemble the League but make sure Superman doesn't go rogue if something happens to Lois.