Complex sentence: Because of the large size of the α-pinenyl groups, diisopinocampheylborane does not hydroborate hindered alkenes but when it does add, it does so with high enantioselectivity, even for small L's.
Split into simpler sentences: Because of the large size of the α-pinenyl groups, diisopinocampheylborane does not hydroborate hindered alkenes. Due to the large size of the two Ipc groups, the addition proceeds with high enantioselectivity, even for small L's.