Any system worth its salt has a wuji posture. Southern mantis for example uses wuji to teach inner sinking without external sinking.K atori shinto ryu does the same with their natural stance, as described by Sugino. A similar description is done by Tom Bisio too. Almost the same as explained in Sun family methods (tcc,xingyi,etc). Dan Harden teaches it as misogi model. Damo mitchell has wuji teachings too...
It's more like if you are only doing stillness in your wuji, you never learned wuji.