RMAN automates Oracle database's backup and recovery processes. If you would like to know what happens inside an RMAN command,(RMAN Internals), then try the debug option. Also, when you have a case opened on any RMAN issue, the debug logfile can help a support analyst quickly locate the source of your RMAN problem.
Its very easy to turn on the debug feature.
/home/oracle/backup_testing> rman log=output.log trace=debug_info.log
RMAN> debug on
RMAN> RMAN Command 1;
RMAN> RMAN Command 2;
The debug_info.log contains all the internal details. BTW, you will not find information about this feature in the Oracle Documentation.