Detailed explanation of the SCIENCE in "ENDGAME" - Heavy Spoilers - Comparison with X-Men

THE ANCIENT ONE (drawing a golden line in the air with the Infinity Stones represented at one point in the line):

The Infinity Stones create what you experience as the flow of time. Remove one of the stones...

(flicks the time stone out of the line)

...and that flow splits.

(A black line splits off from the golden line curving away into the distance)

That may benefit your reality, but my new one, not so much. In this new branched reality, without the chief weapon against the the forces of darkness, our world will be overrun, millions will suffer. Tell me doctor, can your science prevent all that?

BRUCE BANNER:

No, but we can erase it. Because once we've got the stones, we can return it...

(puts the time stone back into the same place in the golden line, and the black line fades to nothing)

...to it's own timeline. So chronologically, in that reality, it never left.

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BRUCE BANNER:

You have to return the stones to the exact moment you got them, or you're gonna add to the deflection of the alternate realities.

STEVE ROGERS:

Don worry Bruce. I'll clip all the branches.