I am just offering a newbie’s thoughts regarding Bagua, but I have been “opening” as the feet reach out and “closing” as the space between the feet narrows.
This is the same sensation I have been trying to get when doing the palm development exercise – when the hands part (opening) vs. when the hands come together (closing). The sensation (as far as I know so far) is that all the major joints (at least, thats all I can try and focus on myself ) open and close at those times. (I assume an experienced person could control the opening/closing of different joints individually, but thats past anything I am working on )
If you meant the kwa open/closing – I have been trying to open the “back” leg AS the weight comes OFF of it.
Hopefully someone more knowledgeable/authoritative will jump in.