The guide wheels are set up in 3 opposing pairs. The Master pair are withdrawn first. Then the Slave pairs are retracted, probably simultaneously. The Micro and circuits allow for any sequence, pulsing or locking/latching to be programmed, but the above seems the most logical. The lever arms holding the wheels have now each been fitted with a  positive locking device. These are 12v car door lock actuators. More circuits have been made to operate these locks. These also contain relays to reverse the current flow. Master and Slave may be referred to elsewhere in these pages as Primary and Secondary.