#sicklyscientist

We know from his experiments that the limit for reversing time is only 4 seconds. Any further back and it causes catastrophic harm to his surroundings. It’s implied that the further he overextends his Z-drive, the more disastrous the damage and radius of effect.

In the final battle against Viktor, we see Ekko overextend his Z-drive far beyond 4 seconds. Then, he yeets it at Viktor, whose mask is shattered from the impact.

We know that turning back time beyond 4 seconds causes catastrophic destruction of the surrounding areas: Jayce and Viktor, and perhaps countless others should have been caught up in the blast. But instead, we see something different:

Time stands still for a moment, courtesy of the boy who shattered time. 

Now things happen all in a manner of seconds: In the astral plane, we see Viktor comment on the anomaly that Ekko has created.

It’s something that Viktor as smart as he is, couldn’t have conceived— leave it to Ekko (and lest I forget, Heimerdinger and alt-Jinx) to show them all up.

This is debatable, but I firmly believe that Viktor with his awareness of all time and space, could have escaped or even survived the explosion.

It’s only Jayce, the only person who could show him what he showed him, that gets through to him in the end and convinces him to give up his Glorious Evolution.

"In all timelines, only you can show me this". Ekko is the catalyst but Jayce was the key.

Jayce returns the Acceleration rune to Viktor which kicks off a chain of events. The souls under Viktor’s control are freed from his grasp. Back in the real world, and back to the explosion, we see the following happening almost simultaneously: 

The blast from the Z-drive extends from Viktor’s reach to Jayce’s

The Acceleration rune embedded in Jayce’s wrist starts to glow from being activated by Jayce and Viktor in the astral plane

Jayce and Viktor are absorbed into the gem and the gemstone blips out of existence 

Recall that the Z-drive was created through an inversion of the Acceleration rune. To multiply something by its inverse, is to divide it by itself. The Acceleration rune multiplied by the Inversed Acceleration rune, cancels out the effect of the other: nullifying both the anomaly from the Z-drive but also the rune.

This explains why there was no larger explosion. Viktor and Jayce (who chose to stay) sacrificed themselves and took themselves out of the equation. Jayce and Viktor were the anomalies— just like the Z-drive should not have existed, neither should have Hextech.

Through all the timelines and permutations of runes that Viktor tried, the only combination that could have worked was in the timeline with the Acceleration rune. It couldn’t have worked if it was any other rune, if Ekko had never invented the Z-drive, or if Jayce had never convinced Viktor to give up his path of destruction, and got his partner back.