I wonder if doing so cancels my HDR 10 bit. HDMI 2.1 is good at 4K up to 120Hz, but if you go beyond that you probably lose out on the 10 bit color depth as it doesn't have the bandwidth.
It does not.
The extra frame or frames just sit in the GPU frame buffer, and then are sent to the monitor when the time comes. As a complete frame.
It has no effect on HDR or Displayport bandwidth.
Possible that the monitor is set to use something lower than 4:4:4 chroma subsampling, using lesser bandwidth and allowing the rest to be used for pushing more frames?
You are thinking about it incorrectly. The monitor does not have a frame buffer, it just displays what is ever coming down the line. All of the framebuffers are on the GPU.
Normal mode:
The monitor constantly updates the screen with whatever is coming down the cable. The framebuffer on the GPU can be updated at any time, resulting in the top part of what is displayed on the monitor coming from frame A, and the bottom part coming from frame B. This can cause tearing.
Vertical-Sync:
What v-sync does is hold the frame in the framebuffer on the GPU, stopping the GPU from updating the frame buffer. This frame is streamed to the monitor unchanged top to bottom to the monitor, and displayed intact. While this is happening the GPU renders the next frame. After rendering the frame, the GPU sits and does nothing, until the monitor finishes its refresh of the screen. It then places the next frame in the GPUs framebuffer in sync with the monitors Vertical Synchronization. Then it begins rendering the next frame. If the GPU is not ready with the next frame when the Vertical Sync time moment occurs, the previous frame in the frame buffer is just streamed to the monitor a second time until the GPU is ready. This caps the FPS on the GPU to the monitors max refresh rate, and decreases frames per second.
FreeSync/Gsync:
The GPU now effectively has multiple "frame buffers". What happens depends on:
A. GPU limited, monitor refresh greater then gpu FPS
--- After having framebuffer A streamed to the monitor, the monitor stops updating and waits for framebuffer B to be ready.
--- The GPU immediately sends framebuffer B to the monitor as soon as it is finished, which the monitor immediately displays as it receives it.
--- While framebuffer B is being sent to the monitor, the GPU begins rendering an new frame into framebuffer A.
--- the two frame buffers basically are swapped back and forth, with the GPU rendering to one, while streaming the contents of the other to the monitor
--- no need to wait to send frames
--- no need to sit and do nothing while waiting for vertical sync
--- frames are displayed on the monitor as soon as they are available, with frames typically not being sent twice
B. Monitor limited, GPU FPS greater then Monitor refresh
--- framebuffer A is stream to the monitor
--- GPU finishes rendering to framebuffer B before framebuffer A is done, GPU starts rendering to framebuffer C
--- Monitor finishes showing framebuffer A, GPU immediately sends it framebuffer B.
--- GPU finishes framebuffer C, framebuffer B still being transferred to monitor
--- GPU finishes framebuffer D, framebuffer B still being transferred to monitor, discards framebuffer C, starts framebuffer E
--- GPU finishes sending framebuffer B to the monitor, starts sending framebuffer D, skipping C, while working on E
--- and so on