On the opposite side to the controls, between the wheels, is a small foot operated lever, which by way of a small cam, engages and disengages the drive - allowing the shovel to free wheel and be propelled along the track by a locomotive or whatever means available.
We have a 12B at Lea Bailey Light Railway - we have the rocker mechanism working (as demonstrated in a YouTube Video:
?list=PLZWKiKiRxGhHCMxGwuTI5LIHSRwUDOjVY) but the propulsion mechanism is still being worked on.
Our Chairman, Rob Needham has a copy of the brochure / instruction manual for the rocker shovel which we were looking at on Sunday. It even has diagrams of how to lay the track inside your mine for efficient operation. I shall PM you his e-mail address so you can request a copy.