Zone of genius in a nutshell is where you happen to get in a flow state by acting through your medium , while also being aligned with your top values, strengths and preferred impact .
For example (me): 
Top 3 values : personal development, understanding, spontaneity 
Top 3 strengths : playfulness, ingenuity, genuineness 
Prefered medium: speaking 
Most meaningful impact: helping men improve their self esteem, self understanding and social skills 
Zone of genius: Coaching social dynamics IRL