Its a manual bombing reticule. you set up your SMS for a manual release, set the depressed reticule to the correct setting in mils for your release profile, and fly your release profile. When the solution is reached (correct speed, altitude and dive angle, reticule on target), pickle to release bombs, and recover to straight and level.
You first need to know the ballistics data for bombs in Falcon, and you have to guesstimate a bit to use it as the depressed reticule is not of fine enough resolution for actual use. Once you have ballistics data for the bombs, you can create a table of depression setting vs dive angle, speed and altitude, to use in order to select a profile before flight.
Given its limitations, its not used much at present. Its there as a backup incase the bombing computer (the MMC) or the HUD fail, and as at present in BMS if the HUD fails both the standard and the backup reticule are not selectable, you cannot use it as a backup in BMS either.
Basically unless you specifically want to make a manual release, there are better options for pretty much every situation in falcon.
EDIT: I understand the Multi Command Handbook included in your BMS install under Docs/Operational Documents has quite a bit of information on manual bombing, which may prove of interest to you.