There is a pretty complete description of the mechanism (or at least one version of it) here:
http://www.lonesentry.com/articles/t...se-bomber.html
The dorsal turret gunner actually had to drop "his" gun and use a pistol grip mechanism to control a ring and bead sight which was connected to the tail gun by linking rods that ran down the sides of the fuselage along with cables to control other functions of the tail gun.
"A 7.7-mm Vickers-type machine gun is mounted in the tail on a free-traversing bracket, the elevation and depression of which is controlled by another bracket attached to the forward end of the gun barrel. Both brackets are connected by link mechanisms to rods running down both sides of the fuselage. These, in turn, are coupled to a curved arm carrying a pistol grip which is operated by the gunner standing in the (dorsal?) turret. Coupled in the linkage is a ring-and-bead sight, which protrudes through two holes in the turret, and movement of the pistol grip controls the gun and the sight. The maximum movement appears to be 10 degrees in any direction, giving a cone of fire with a 20-degree angle. On the control arm are two "T" handles connected by Bowden cable to the cocking handle and to the stoppage clearing handle, respectively. The gun is fired by a trigger mechanism in the arm, connected by Bowden cable to the front sear on the gun."
An explanation of the Bowden cable is here.
http://en.wikipedia.org/wiki/Bowden_cable
All in all it would appear to be a totally mechanical mechanism with no electrical components at all. Apparently there was no "connection" to the turret gun at all. The turret gunner had to switch between the two "guns" to operate them both. probably not very easy to do when under attack.