Leica M5 speed dial question

The speed dial of my M5 has double number B1 , which means it could be speed B or speed 1 sec.

This is rather confusing. In what circumstances the shutter dial at B1 acts like B, when acts like 1 sec?

On M6, B and 1 are separated.

The speed dial of my M5 has double number B1 , which means it could be speed B or speed 1 sec.

 

This is rather confusing. In what circumstances the shutter dial at B1 acts like B, when acts like 1 sec?

The 1 sec through 30 sec times (B1, B2, B4 etc.) must be timed by the photographer. From the M5 manual:

"The speed dial is additionally engraved with 'B' (bulb) from 1 to 30 seconds, for use with the light meter. These exposure intervals must be timed by the photographer, and a cable release should, of course, be used. Starting at the interval marked 'B1', the shutter will remain open as long as the cable release is held down."