This link explains how the lock worked and discusses picking techniques, though not Hobbs' methods specifically. I vaguely remember reading that his equipment included a device to apply a small torque to the cylinder, via a lever arm with a short pendulum on its end. Any movement of the cylinder as a slot aligned with the shear line ring was revealed by the swinging of the pendulum.

The use of shallow decoy slots indicate that Bramah had anticipated this form of attack, but they only served to delay the solution.

http://crypto.com/photos/misc/bramah/