It’s just a crazy tool. You can drag the ends around and it keeps smearing new mask out like a wet string dipped in ink. Useless. It sort of snakes around the features physically too. Does NOT fill in the center of a loop, nor acknowledge that the loop is closed. The icon is thus misleading since it uses the same icon as the lasso tool, showing a filled interior.
Draw Size does control how thick the masking line becomes when you Control-click the line, but the result tapers off at the start of the curve and gets lighter unless you then also grab and jiggle the end point to make the whole snake move a bit back and forth.
This brush isn’t even in the online manual so seems to have been abandoned. I wish I had bezier editable masking loops after all.