This is a controversial subject when it really shouldn't be.
It's just a tool.
Like figural representation in painting, the rule of thirds in photography, or the hero's journey in screenwriting - it's an idea that's better to have in your toolbox, than not.
You can use it, you can throw it away, you can keep what you like.
The idea is simple:
There is an imaginary line coming out of a subject, splitting the person in half.
There's the left half, and the right half.
When filming a person, especially in a documentary setting, when you film one side of the line, it's good hygiene to film the reverse from the same side of the line, as pictured:
If you want (or need) to jump the line to capture something - go ahead!
This is not meant to be a prison for your videography.
It's simply a way to capture satisfying, continuous, editorially-sound footage.