With the new leaks of the next chapter, I've come to this conclusion:
The trigger for Fyodor's ability would simply be death!
That's why he made everyone believe it was the touch..
Bcs Dazai is the only one to kill him, as I imagine Fyodor's ability would be to be immortal and only activate when he dies. That's why he lied about his trigger, to avoid Dazai touching him while he was dying!