Disiamylborane is prepared by hydroboration of trimethylethylene with diborane. The reaction stops at the secondary borane due to steric hindrance.
Disiamylborane is relatively selective for terminal alkynes and alkenes vs internal alkynes and alkenes. Like most hydroboration, the addition proceeds in an anti-Markovnikov manner.  It can be used to convert terminal alkynes, into aldehydes.
The hydroboration process proceeds via an initial dissociation of the dimer.