What makes a nice bum-swing on a woman..?

Effort. Swing is generally intentional and most women really don't want you staring at their ass.

Because women aren't all the same?

I’d assume practice.

Genetics, diet, exercise

Well if you are being serious, it is the same for all people. It all comes down to peak kinetic biomechanics. A truly elite "bum-swing" isn't just random movement, it is a finely tuned performance of highly specific musculature.

The Gluteus Oscillatoris is located deep within the pelvic core and this critical muscle acts like an internal pendulum, creating a perfectly rhythmic side-to-side momentum with zero wasted friction. Bipedal Strut-Tenson when anchored along the upper thigh stabilizes the stride so the swing achieves maximum sway without throwing off forward velocity. Tensing the Sastimus Maximus is the primary driver of attitude. Rotational Quadricep thrust is responsible for that subtle, effortless flick at the peak of every step that makes the whole motion look entirely unbothered. Mathematically speaking, it’s just pure physics.

Or not. Cheers!

I don’t even know why people bother commenting, cringe post that will be deleted by end of day