I believe he is saying that traditional metrics of mastery such as the "10,000 hours" of work metric are incomplete because they are extrinsically defined metrics. Venkatesh makes the point that except for a few well defined fields such as mathematics or violin (fields with predictable boundedness), such extrinsically defined metrics do not provide much useful information.

Therefore, using the analogy of relativity, Rao proposes a new metric of mastery based on intrinsic metrics. Metrics that he believes may provide insight as to whether you are truly improving if your work spans that of multiple fields. These metrics function as axes for your journey through "endeavor space". These axes are rework, referencing, and releasing. If you are wondering whether you working with grit or wasting your time, a self evaluation via these axes can help to tell you whether you are working towards mastery or not.