It is well known that the Leica M's focal-plane shutter causes little vibration when it fires. But for lengthy exposure times, a tripod and Cable Release are essential. The remote release for the Leica M, featuring a conical thread, is screwed into the shutter release bushing. A locking screw lets you make long time exposures at the B setting.
For exposures longer than four seconds using the Leica M7 for example, set the shutter-speed dial to B and, after firing via the remote release, hold the shutter in the open position for the desired exposure using its locking screw.