How did takemitchy time leaped for the first time????? Getting hit by the train was the trigger???( I have Completed the manga)
no that wasnt the trigger as to what was the trigger it is that if both users have an insanely strong will to change the past then tackemichy can leap
The first trigger was probably his will, regrets and maybe his death as well.
He helps Naoto with the delinquents, finds the resolve to change the future, talks to Nato about it and then shakes his hand. This is probably when the trigger changed to the current one.
Because of this the future changes and Naoto survives & saves Takemichi.
Though this is just a really simple "theory", we weren't given an explanation to how he first changed the past. We just know when and why the power changed thanks to Naoto.