I agree with JJ that good sway is easier to prescribe than it is to do.
The instruction "from the ankles upwards" is, however, a good instruction, and it stops in their tracks anyone who believes that sway comes primarily from the shoulder line (or at best, the waist).
Done to the right degree (in turning figures in social dancing, at least), I think it is the couple themselves who will be mainly aware of their sway - an inclination of really the whole body to the inside of the turn, to counteract centrifugal force - and observers will see very little in the way of "styling". If that much sway is proportionaly increased, so that it is readily apparent to an observer, then that is about as much as would ever be required. The rise and fall is more important, and in one sense, the use of sway can be likened to modifed rise, in that you are stretching one side of the body more than the other.
If it looks contrived, it is either being overdone, or being done wrongly.