I get that but if you twist, the base is no long flat. The edge does not have to be engaged and is irrelevant. The base is not flat. Go ahead and twist the base on a box. Even though you never come close to actually tilting onto the edge, you will se daylight under some portion of a twisted base. My point remains valid that if you apply enough pressure to twist the base it is no longer truly flat ergo not truly flat basing. To truly flat base a board, use pivot and pressure not twist.
Ok. In my book it's close enough to flat and the twisting isn't used all the time, it's only used for small steering corrections. If the snow is soft, you won't see daylight. A very small amount of twist is enough.
If the snow is hard, I think what matters is that you are engaging the edge/sidecut.