Sync AC input signal with the Clock signal

The AC input waveform is lined up with the Inverted Clock signal

Oscilloscope channel#1 is AC input
Oscilloscope channel#2 is Inverted Trigger Pulse signal



I think this is called sync right?

I'm not sure as to why the designer wants up to align the inverted clock signal triggered pulse with the inputs AC waveform, do you know why?

The AC sine waveform is moves from left to right when adjusting it with R10 100K pot U3 is a 555 timer
Adjust the 100K pot of U3 with shift the AC sine waveform from left to right

U9 is a 555 timer also, R33 100K pot adjust the pulse width of the inverted square waveform output
I adjust the pulse width to fit the AC sine waveform

U9 only puts out a squarewaveform when it is triggered, the trigger signal is very short and quick

I'm not sure if we are syncing up the AC input signal with the clock signal which is U3 or the trigger pulse of U9

Here is parts of the schematic:





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I can't get the channel#2 trigger pulse to happen using "AUTO trigger mode" on the oscilloscope, So I have to use the oscilloscope in "NORMAL mode". I have to adjust the Trim Pots using Normal mode , but you can't see the difference until you hit auto trigger mode and hit normal mode again to see if the AC waveforms input is in sync with the inverted trigger pulse on channel#2. Is there any other way to use to oscilloscope to sync up these waveforms besides using it in Normal mode for triggering?

The Sine waveform has to fit inside the 10milli-second pulse width

What is this called? Is this syncing up the input with the trigger pulse?

Phase Locking Test:
Oscilloscope channel#1 on the input on unit which has AC sinewave as the input to the unit

Oscilloscope Channel#2 is on the output of the unit which is AC waveform

To Check the Phase Lag, you move the AC waveforms aligning them together

If the input and output sinewaveform is zero degrees the Phase Lock is GOOD? if the input and output sine waveform is Off phase, what is wrong with the circuit? What cause the input and output to be out of phase between %5 to 175%?