I was reading some of Ted Greene's work and came across this PDF
In it, he goes from a vi6 chord (this 6 doesn't mean first inversion, it means the chord with an added major sixth, in this case spelled tonic-major sixth-minor third-fifth) to a I chord. I.e. from a Cm6 chord (spelled C-A-Eb-G) to Eb (spelled Bb-Bb-Eb-G).
I can't understand how this makes sense in the context of tonal harmony. Any help?