Suppose that you jounce up and down while weighing yourself on a bathroom scale. which varies: the upward support force or the force of gravity on you? why is your weight reading best shown when you stand at rest on the scale?

The bathroom scale always measures the normal force that acts on the body when placed on it. The body stores energy which is transformed into force when a displacement is made. This extra force should be added or subtracted from the weight.

N = mg ± F

Keep in mind that the bathroom scale is only responsible for displaying Normal Force. Therefore when jumping up or moving down the bathroom scale will indicate a different value because the normal force acting on the body is varying: There is weight and internal Force.

This does not happen in full equilibrium condition when the normal force is equal to the weight. In that condition the balance would indicate the correct weight.

N = mg

none, its a machine