[eschatology]

My understanding is that on the darker scenes (say, 0 to 5 in the brightness slider example), there is difference in luminance value with HDR but not SDR, so there is increased contrast and detail.

This matches my experience; 0 to 5 look identically black if I turn off HDR

[kllrnohj]

You may have a monitor that only enables local dimming zones when fed an HDR signal but not when fed an SDR one, but that would be unusual and certainly not required. And likely something you could change in your monitors controls. On things like an OLED, though, there's no difference in the darks. You'd see a difference between 8bit and 10bit potentially depending on what "0 to 5" means, but 10-bit SDR is absolutely a thing (it predates HDR even)

But like if you can't see a difference between 0 to 5 in a test pattern like this https://images.app.goo.gl/WY3FhCB1okaRANc28 in SDR but you can in HDR then that just means your SDR factory calibration is bad, or you've fiddled with settings that broke it.