How do Guard Rails work from a programmer point of view?

I understand what Guard rails do. I want to know how I code them. The explanations I have read are all quite high level and treat Guard Rails as something of a black box. What do I need to know to try developing some example Guard Rails? submitted by /u/Richard210363 <a href="https://www.reddit.com/r/artificial/comments/1sl56q5/how_do_guard_ra