I've often been told to 'bend at the waist'. But I cherish my back too much to follow such misguidance, choosing instead to bend at the hip joints.
If you peruse Youtube videos on human anatomy, you'll quickly see how the human body is much more adept at bending at the hip joints than at the waist. The hip joints attach the femurs or upper legs to the underside of the pelvis, and are designed to flex over a wide range in three aspects---side to side, ahead and back, and rotationally---with movements powered by a network of fascia, ligaments and big muscles. The waist, which circles the trunk just above the pelvis, means that 'bend at the waist' requires bending and/or twisting the spine. When skiing, we are subjected to large dynamic forces that act to pull our body downwards towards our feet, just as when we lift or carry heavy weights. Workmen's compensation statistics prove that bending or twisting the spine while lifting heavy weights is a bad idea, often resulting in back injuries.
Watch this Youtube video: The waist is not a joint
Modern advice on body mechanics eschews bending at the waist, so why do I still hear it so often in ski teaching circles? I think it may be a throwback to a time when body mechanics were less widely considered, or, it may just be easier to say 'bend at the waist' rather than 'bend at the hip joints'. Suggestion: follow and teach bending at the hip joints and save the world some unnecessary back pain.
Further on this topic, I like the notion that the upper body, a significant portion of our body mass, becomes a 'foundation' for leg movements particularly when skiing at moderate to high speeds. More on this topic in Ski Well Simply.
Angles at the hip joint establish the geometry of our stance and greatly affect the reaction of our skis.