A sinking mode:
And now she makes a mental image of swimming downhill:
How to think about 'sinking hips" --
A strong pull of the hand through the water, aimed directly at the front of the hips, helps to create "lift" under hips - like the wind lift under the wing of an airplane.
By slightly tucking the tailbone under (it's more of a release of tailbone to lengthened position, not a tense grip), you can create a better shape to capture the lift created from the turbulent water below the hips.
Swimming "more downhill" helps tip body balance in the water: by adding weight toward the top half of the body, moving the cernter of gravity "out of the hips" and "over the lungs." The arms, though not particularly heavy, add weight by delaying their "pull" (this keeps one hand "out front, holding the water" until the latest possible moment). A relaxed neck allows a deep head position and arms should reach deep into the water with each stroke.
One more thought: if the hips roll to perpendicular, the bowl-like shape holding the turbulent water is "broken" and the hips will sink. To keep their "aerodynamic" shape they must still have some of their horizontal position. This means that shoulders are rolling open more then the hips - a natural bipedal twisting motion occurs at the waist. More about "biped motion" later!