synchros

One output (say output A) of the synchro would look like this on an oscilloscope:

... and it would look the same whichever way the shaft was rotating.

The second output (B, in pink, below) gives this result for, say, clockwise rotation:

Note that the B output (in pink), is rising before the A output (in blue), as the angle of rotation increases (left to right).

With anticlockwise rotation, the outputs change round, so now the A output (blue) rises before the B output (pink):

The sensing electronics can use this information to indicate the direction of rotation.