When doing “rocking” exercises on the stomach (rocking, swan, etc.), imagine the rails of a rocking horse, or rocking chair. Uniform, symmetrical shape is beneficial to the technique.
The uniformity of the vertebrae is basic in concept, but, for many, difficult in execution.
The real key is forcing the abdominal wall to behave like the hard rails of the chair or horsie. After this, it’s a matter of finding the stretch in opposition for the perfect elongation of the vertebrae, as you attempt to mold the perfect arc (weld).
So, keep on rockin’!