Hey guys!
The only thing I can say about how the algorithm works is:
"Take an internal face, compute the volume of the cell on either side and divide the bigger by the smaller".
High volume ratio often correlates with a high aspect ratio. OpenFOAM will call everything below 100 “good” (checkMesh reports the inverse number, though, so it’ll tell you that everything > 0.01 is “good”).
Cheers!
Jousef