This is why when your opponent tries to move to your left "side door" (blind side), he should move his back foot first. When he moves his back foot, the distance between you and him has not changed yet. If you can't reach to his leading leg before, you still can't reach to his leading leg now. If he then quickly moves his leading foot, he can get the "angle" that he wants and reduces his risk to the minimum.
I had a serious argument on this subject with Bagua people before. When a Bagua guy tries to move toward your "side door", he will move his leading leg first. If you charge in while he is moving his leading leg, you can crash him down.
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if you know chi gurk (assuming you have done the training to apply it) and someone crosses their legs up like that they should be finished.