Nothing magical; just front and back of the midpoint between the eyebrows until you settle on a point where you get the sensation. That is why i called it a "physiological trick".
In the exercise i mentioned, you physically focus the eyes at a point in the blackness and the mental focus/attention follows it simultaneously.
The basic idea is that the space around your body to the maximum distance reachable by your arms/legs called "peripersonal space" (https://en.wikipedia.org/wiki/Michael_Graziano#Peripersonal_...) is integrated into a larger body schema maintained (partially) in the somatotopic map and its homunculi. Proprioception is part but not whole of it and how it might be regulated by higher executive functions like focus/attention is AFAIK not clear.