@Shinigamisama You're confusing "death Flag" with the Shonen Trop "training arc trigger".
It's the moment something happens to push the MC to get stronger like never before. If they lacked a motivation before then they get one, their motivation is reinforced, or it's replaced. In any case they are given a stronger reason to get stronger. What generally follows is either some kind of journey to get stronger, spartan training, or just asking for help from someone another cast member didn't want to owe a favor but is able to make them stronger.
A death flag would be something like "don't worry after this is over I'll answer your confession" or something like that.