The short explanation
HDMI carries a +5V line that says “something is connected here”. The display answers with a hot-plug detect signal, and that exchange is how a device knows a display is present. Force it off and the device stops seeing the downstream display at all.
Where you run into it
That sounds like sabotage, and used carelessly it is. Used deliberately it is a diagnostic and a fix. It stops a display that sleeps, wakes slowly or drops out from disturbing the upstream negotiation, and it lets an emulated EDID stand entirely on its own rather than tracking the real sink.
It is also a clean test: if a fault disappears once the display is invisible, the display's own behavior was part of the problem.
How the DR HDMI 8K handles it
TX +5V force is on by default on the DR HDMI 8K. Setting it to off means the display at the output is not detected by the unit, as if the HDMI cable were physically disconnected. Combined with a fixed custom EDID, that gives you a source that sees exactly one stable display and nothing else.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.