In Chiang Jung Chiao’s Bagua, the Bai Bu is also considered a kick. It would be a sort of sideways stomp kick to the knee or a stomp to the foot then.
But it can also be used to attack someone’s knee with your knee. You might deflect a punch on the closed side, stepping acroos with a bai bu. If you plant your foot right near his, so that your knees are almost touching, and then bend your knee against his, you can break his root and tumble him over. You have to be careful practicing something like that, though, because you can easily hurt your partner’s knee.
“I put forth my power and he was broken.
I withdrew my power and he was ground into fine dust.”
-Aleister Crowley, The Vision and the Voice