I do not understand how the stereoselectivity arises. We have a chiral boronate, true, and we have facial selectivity on addition to boron centre. But I don't understand why one face is selected over the other - With one ester group on the ring, I could see that would sterically block one face of the boron centre, but with 2 ester groups it seems that each side of the boron is equally blocked.
This figure from this reference (p. 170). might help you visualize things.
As you can see, the stereochemistry is given by electronic repulsion between the carbonyl group and the incoming aldehyde.