Phil, an advance over the 12/24 way.
Basically a fiendish bomb selector the can be used to select single bombs, some bombs, all the bombs etc.
On a Vulcan for instance the 1000 lb bombs were in two layers, 12 at the top 9 at the bottom. You could chose any one of those 9 for single release etc.
It also allowed you select the drop interval between bombs.
The standard mantra was Zero Press, Zero Press which ensured that the bomb release signal was always sent to the first available bomb. For a single bomb,IIRC, you would set the Stop to One.
*note to self, visit Newark for a refresher.