Asymmetric hydroboration

mycotheology

So I'm reading about asymmetric synthesis and read that a commonly used "asymmetric reagent" is this:

thats the dimer you get when you react alpha-pinene with borane. If I'm not mistaken, the reason this reaction doesn't go all the way and produce a trimer is because there is too much steric hindrance for a third a-pinene molecule to fit in there.

I'm trying to understand how this compound is used for stereoselective additions to alkenes. I'm guessing that the alkene has to react with the boron and form a trimer (by replacing the hydrogen). Firstly does that mean that this will only work on small alkenes because steric hindrance will block bigger alkenes from reacting with the boron?

My main question is how exactly does this make the hydroboration reaction stereoselective? Is it purely sterics? In other words, do the big bulky pinocampheol groups make it so the boron can only approach from the less hindered side of the alkene?

chemisttree

Stereoselective or regioselective?