Use 'you know who' when you want to avoid saying someone's name, usually because it's sensitive or obvious. I can't believe what happened with - you know who - yesterday.
Pro Tip 2/3
Shared Knowledge
'You know who' assumes the listener knows who you're talking about. Don't use it with strangers. Every Christmas, you know who loves to wear that red sweater.
Pro Tip 3/3
Hiding Identity
Use this phrase to discreetly discuss someone, often to keep their identity private in public. I heard you know who got promoted.