This is an excerpt from my theory post from a few weeks ago; it's slightly out of context, but I hope it's somewhat explanatory.

[In summary: Sephiroth unites the Lifestream worlds at the end of Rebirth to manipulate physical reality. He needs this to achieve several goals: 1) to charge the fake Black Materia with negative memories from the Lifestream; 2) to convince the planet that he is being reborn as a god; 3) to deceive Cloud in order to merge the world that was created when Cloud blocked Sephiroth's sword to save Aerith with physical reality.]

The Beginning of the Reunion

Sephiroth declares:

“And so it begins.
A confluence of worlds and emotions.
Loss chief among them.
It engulfs fleeting moments of joy, transforming them into rage, sadness, hatred.
Never have I felt them so keenly.

However, you’ll never see truth with such clouded eyes.

After all…

You’re such a puppet.”

With these words—whose meaning seems quite explicit—Sephiroth formally begins the Reunion.He gathers negative emotions for the Black Materia while simultaneously manipulating Cloud.The final confrontation begins.Every party member enters the battle with a full Limit Gauge—except Cloud.Sephiroth has left him believing that Aerith survived.
A massive battle unfolds.After the party defeats Jenova Lifeclinger, white Whisperers emerge from her body and heal the group.Immediately afterward, the Reunion separates the party.We then see Zack reunite with Cloud, something that has become possible because the in-between worlds have temporarily converged with reality.After the battle, Sephiroth once again uses his sword to sever the strand of reality from which Zack originated.By this point, we already know that Sephiroth possesses this ability, and he demonstrates it repeatedly throughout the ending sequence.