I think he is using a straight bar. bent at ends.
thumbs in would be pulling with upper back/lats
thumbs out - depending on grip width can hit mid to hi back, and bi's cause you are pulling.
Wait, I just reread... I would recommend you use a straight bar instead.
Sounds like you are using a triangle? or a double stirrup?
Check out this place for different attachments, then we can make a better assesment,