This needs you to simplify, to draw metaphors that can help the other person relate to what you are saying, both of which requires a deep understanding from your end in the first place. 

If your understanding of a topic is superficial, you will fail at this step. 

But, an even stronger indicator of how well you know something is your ability to convince someone else who has a different understanding to change their mind about it and to see it your way. 

This needs you to not just simplify and draw metaphors, but potentially also question your own assumptions and to consider conflicting view points that you might have otherwise missed. 

The best way to learn about anything is to debate the topic.