I was checking up on some squash rules and realised that I have never really grasped the difference between stroke and let in squash. Especially if there is no dedicated ref, how do you go about deciding these?

[Here's an interesting video](https://www.youtube.com/watch?v=tiUKarLetAA), where three events: let, stroke and no-let happen during some match, but they looked pretty much the same to me. Could someone explain why the calls are made in that way?