Kick-down is used when maximum acceleration is needed, such as for overtaking. When the accelerator pedal is pressed all the way to the floor (beyond the position normally regarded as full acceleration) a lower gear is immediately engaged.

Is kickdown bad for the car?

No, unless the engine speed increases excessively (in an emergency you will accept this for braking). Just about any vehicle will deny kickdown if your road speed is too high "at that time". It will put stress on your drivetrain, but this should be well within design parameters.

When should you down shift automatic transmission?

When should you downshift automatic transmissions? You can select a low range to get greater engine braking when going down grades. The lower ranges prevent the transmission from shifting up beyond the selected gear (unless the governor rpm is exceeded).