Can someone explain the clock sync?

I would not worry about any of the clock stuff. The whole jitter-worry is overhyped as with anything remotely contemporary, jitter is not audible at the values found in contemporary digital devices. See here for a discussion:

https://audioxpress.com/article/audio-electronics-is-digital-jitter-really-a-problem

You don't worry about MHz output in the audio signal, as it is equally irrelevant. If you hear differences in DACs, it may be a whole lot of things before I would consider jitter.
Also the video linked above explains nothing about what is supposed to happen with this clock stuff, just how to do settings on a particular device.

Just enjoy the music!

@fthompson251 

This information from the Denafrips site should help

In short, your DDC reclocks the signal from your streamer before sending it down to the DAC. There’s also DSP that takes place. The objective is to lower jitter to levels where it is no longet a factor for the DAC. The i2s into the DAC is then extremely clean. This supposed to result in better sound. 

My understanding is clock synching between two devices has the clocks work together seamlessly when clocking signal inputs, so the signals the Hermes are fed are clocked in tandem with the clock in the Terminator. IOW the two devices "are on the same page". This synching supposedly lowers jitter thus distortion, sonically speaking this supposedly improves phase coherence thus clarity. In my system this benefit has been mercifully clear, but often others report little to zero audible improvement. Congrats on the Terminator 15th, curious to know if the rumor is true that it's a rebadged Plus.

Hey OP,

The idea behind a master clock comes from the pro world where you might have many digital sources that had to be kept in synchronization. Especially true for movie production. You can’t have skew.

The only time skew is really an issue for home AFAIK is in a situation where you have an external CD player and separate DAC. A master clock will ensure the CD player gives the DAC bits just in time.

The thing audiophiles and vendors try to do by spending more money is reducing jitter figures. That is, ensuring that the width of each step in the signal remains exactly the same. Not something external clocks were meant to do, but they have some possible uses.

The lowest phase error is now going to be by putting a very good (femto-) clock right next to the DAC. The longer the distance between the clock and the DAC the more error you can have, so having a clock and cable plus internal distance travel means the internal clock MAY outperform any external clock. Benchmark has a good white paper on this and Mytek also wrote about this.

Probably the best idea for home would be to use the DAC’s clock as the master, have a buffer, and sync external sources to it. Asynch USB DACs do this, but some also have clock out capability, allowing the DAC clock to control an external CD spinner. Combination DAC/Streamers do a similar thing but usually have up to several seconds of buffering. Roon does something like this in their server.

Before Async USB DAC’s attempted to fix these issues by guessing the incoming clock rate and buffering the input. They still do this when needed such as for S/PDIF inputs.

Also, lastly, in the last 10 years or so chip clocks have gotten SO much better than they were before. Unless your DAC is old the internal clock is probably going to be the best reference.

Thank you @kairosman ​@audphile1 ​​​​@erik_squires for your responses.