From a body mechanics POV, you are correct that this is a very vulnerable position that should be used with great care. Even as a quick transitional movement, anyone with achy knees can relate to your concern.
My only knowledge of Hsing I (Xing Yi) and Bagua are from the Jet Li movie "The One" , but whenever I have seen this movement in a form or technique there is a high strike used for distraction/set-up or there is a lowering of the center of balance, twisting into a kneeling stance, that would serve to protect the knee and add springiness to the follow up technique.
Every offensive technique creates an opening for a counter. In this case keeping the knee bent and at an angle where any direct attack would hit behind the knee and cause it to fold and not break is the way to go. Is that how your technique progresses?
There is a great amount of torque power generated with these techniques, so I will continue to practice them, but caution is the key.
Be well and train hard.