Those three words are the keys to defeat the Sword Saint. I realized it after finally beating him. No any other guide can give you a better advice than Isshin himself. I don't know if Fromsoftware planned it to be a hint. ( My money on it is.)
The thing is you can try to just run around the field to avoid his attack and die. Or you can go defend and die losing posture. The only way to victory and is not to hesitate and attack. I realized it with his second phase. There are no way you can deplete his health down and get a deathblow. And he has a quick posture cool down. And I decided not to hesitate and fight with him. Attack, parry parry mikiri attack and jump. That's it.
I struggled with the second phase for so long. I was running around like a chicken and got killed. Then after hours I again decided to heed his advice and not to hesitate. He is fast but i didn't hesitate. And I did it.
By the time I got into third phase I did heed only to the old mans advice. I didn't hesitate. I attacked and parried and reversed light. I only entered third phase once and it's over.
Tldr: hesitation is defeat.