Select Subject - How to Subtract ?

A few comments in no particular order:

• You have "Flow" set quite low, for your Brush - the effect of brushing may not be so instantly obvious, as when that was high or maximum.
• Whenever brushing will have the effect of Subtract you should see a minus icon at the brush indicator centre. IOW the more you brush on, the more subtracted areas you are making. You may be accustomed to holding down a key modifier (Alt on Windows, I assume Option on Mac) for erasing from - reducing the extent of - a standard brushed mask: the kind that adds adjustment wherever you've painted. But in this case you don't want that key modifier, because this mask is in effect removing adjustment . You'd want that key modifier only to correct if you'd brushed too much subtraction on (the brush centre then changing temporarily to a plus icon, since that erasure would be allowing selection to happen again, in those places).
• I can't quite see in your video whether it is checked, but Automask may get in the way of achieving a clean area of removal. Probably useful just along the boundary of the rock where that adjoins the Subject selection parts you want to keep.