Well, now you’ve confused and lost me ??? If your description is accurate, which I doubt, than it indeed contradicts bagua principles. Without knowing what the teacher is teaching and why you are doing it, it is a waste of time.
Let’s start from the bottom. In my experience, when someone who is Chinese discuss the waist, they are usually referring to much more than the belt line. It implies the entire middle section from the kua to the dan tien and around the back to the ming men. How can you turn your waist separately from the hips?
The waist allows for an infinite number of motions from turning to rotating and from opening to closing. Any motion from the waist will send energy through the entire body whether or not it is perceptive from the outside. Whether you see the leg move or not, there is a wave of energy that travels down to the foot. It is not uncommon to have bagua training that is stationary. That is to say without steps but hardly static. It would be uncommon to not have the whole body connected.
In bagua, 3 of the ten main principles are to raise your back, relax your shoulders, and sink your elbows. To “tense your shoulder and raise your elbow” restricts the flow of energy and limits your ability to open and close the joints for issuing power. As far as my knowledge of anatomy goes, you can strengthen muscle, you can strengthen tendon, you can strengthen bone, but the joint is not something that can’t be strengthened. It’s only a space where to bones meet. In bagua, it is necessary to open and close the joints and increase the range of motion around the joints. In order to twist the full range of motion you can not be rigid and tense.
There are many drills in bagua that require the feet remain parallel and the waist is turning. The arms go through various swings. The techniques of rolling, turning, overturning, wrapping, thrusting, piercing, drilling etc, etc, follow the motion of the “waist”. But they clearly require whole body integration and adherence to the main bagua principles. This includes the sinkings, the pressings, the twistings, the openings, etc.
One of the most important things in learning bagua, (or any martial art), is to know why you are doing something. If your teacher can’t or won’t answer a question about this, either they don’t know or don’t want to teach you. Either way, I would look for another teacher. I would never criticize a teacher or a school that I don’t have first hand experience with. It is a relatively small community, especially in bagua teachers that follow the lineage of Yin Fu. I only asked about your school to get some idea of the level of the teacher, not to say they don’t know what they are doing. If I don’t personally know your teacher, I only have your word and that doesn’t reflect anything beyond your level of understanding. But I will say, if the people and/or teacher in a school get “paranoid” when demonstrating their bagua and their training methods in front of other students or the public, this is an indicator that they don’t know what they are doing or why.