Thread:Zeldakasumi/@comment-27001444-20151014050645/@comment-27281875-20160103083420

"SERN traced the D-Mail about Kurisu's Death which was sent by Okabe and accidentally sent to Daru's phone in the past" is what leads to Okabe's Reading Steiner to trigger in the beginning of the story. Alpha worldline is where that thing happened. Cancelling those will make Okabe return to where he belongs in the first place, the Beta Worldline.

Reading Steiner is not just simply active by "Sending a D-Mail", any acts that could alter past, present, and future, can lead Reading Steiner to active (this might be proven more in Steins;Gate 0, where okabe did not do anything to alter the time but only making simple choice and Reading Steiner kicks in). Reading Steiner is Okabe's ability to observe that something changed in the past and present.