The commanding position
Bed and desk placed where you can see the door without sitting in its path. The tradition calls it command. Prospect and refuge, in environmental psychology, describes the same preference: a wide view from a protected back. Either way the room asks less of you, and calm comes more easily.
What prospect and refuge means
Appleton proposed in 1975 that people prefer places offering a wide outlook and a protected back, the pairing that once decided whether you saw a thing before it saw you. The preference shows up reliably in landscape and interior studies. Feng shui arrived at the same arrangement by a different road, centuries earlier.
Diagonal, not facing
Straight across from the door puts you in the traffic and the draught. Diagonal keeps the view without the exposure, which is why the tradition specifies the far corner rather than the far wall.
For a desk, this is the one that matters
A screen you can be read over your shoulder splits attention whether or not anyone is there. If the desk cannot move, a mirror propped so the door appears in your peripheral vision restores the view, and it is the standard fix in the tradition as well.
Where it stops being useful
This is a placement rule, not a treatment. It costs nothing and the room feels better arranged this way, which is the whole claim. Graded plausible because the psychology is real and the specific effect of moving your own desk has not been trialled.