Stereoselectivity of Roush crotylboration

One can use a tartrate derived boronate to enact enantioselective crotyl transfer. Eg in scheme below:
[![enter image description here](https://i.stack.imgur.com/SnYCt.jpg)](https://i.stack.imgur.com/SnYCt.jpg)


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).](https://books.google.com/books?isbn=0471465240) might help you visualize things. [![enter image description here](https://i.stack.imgur.com/CaTpI.jpg)](https://i.stack.imgur.com/CaTpI.jpg)


As you can see, the stereochemistry is given by electronic repulsion between the carbonyl group and the incoming aldehyde.


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