when does pre cum happen?

Rina • Hey glow friends! My name is Rina M. And I`m 25 year.I have a 6 year old daughter. We love to live life to the fullest!
I've and read that pre cum can make you pregnant but when exactly does pre cum happen? It's a bit tmi but I'm curious to know because I've been using the withdrawal method for going on 4 years with my partner and I'm curious to know when and how does the pre cum happen?