Using Momentum for HEC-RAS Junctions

Could someone shed some light on the add friction and add weight options for the momentum equation in the Junction editor of HEC-RAS. The HEC-RAS reference manual is unclear on these options and I was hoping that someone on the HEC-USERS list could help me understand these two option better.

Any and all comments will be appreciated.
Thanks in advance.
Andrew N. Sauer
SauerAN@cdm.com