can anyone explain how fiddlesticks passive fearing works?

I've had times i hit people while in it and they get feared and times they dont, its especially inconsistent when i ult from passive. any ideas? as a side note, im trying to learn fiddle but cant seem to do well on him. ive been playing league since 2016 but hes just so different, i cant figure out what im doing wrong, i just keep dying with him and not fearing much. also how should i use my trinkets?