It's more than just relaxing a few hard rules. It is also about knowing why you, as a writer, plan to relax or break a rule. Is it readability? Is it variety? Is if for impact of a statement or idea?

We are taught to write formally so that we can express our thoughts and ideas clearly without the reader present. After learning that, writers can opt to relax some of the rules of writing because knowing a rule and why you break it separates you from those who write poorly without any care for the rules.