bsub is a script written by Neal Adams which calls the "real" executable:
/afs/slac/package/lsf/curr/bin/bsubx
Depending on the "-a" option (for suncat this is typically "openmpi") bsubx calls an "esub" script (in the lfs "etc" directory). This in turn points to another wrapper script in the lsf "bin" directory. For openmpi the script is "openmpirun_wrapper". This last one is the one that executes the mpirun command. mpirun uses "lsgrun" on the master node to direct the "res" daemons on the slave nodes to start executables.