You can think of the input as a sin(x) between 0 and 360. TDC is at the 0 point at 180.

Only the first half, 0 - 180 is of interest and using a certain voltage threshold I think of it as a square wave. The trailing edge is in front of TDC and given the amplitude and frequency varies, the amount of advance can be variable.

The change was to make the trailing edge coincide with the zero point ( at least for the first pulses for starting )