the last stage of the transistor amplifier drives the two mag amps, which are cross connected so that one runs the motor in one direction while the other runs the motor in the opposite direction.
depending on when the transistor turns on (due to the phase of the signal from the autosyns) it will turn on one or the other mag amp.
(the schematic here might have swapped wires on the mag amp outputs, so ignore that)
to sum up: the whole gear train (which is all connected together) gets rotated by the motor so that both the coarse and fine autosyns match the input signals coming from the external sensor's pair of coarse and fine synchro senders.