With ATHR in (normal ops) the aircraft will fly at Vapp Target all the way down. With a tail wind this will be Vapp ( + no 'gust' value) because there is a tailwind in this instance. As we enter ground effect the tail wind will typically reduce giving an increase in our airspeed. Either way we will have a higher groundspeed. To counter this, closing the thrust levers and using the flare to control the speed will help ensure a touch down in the right place and with a 'normal' touch. If you retard the thrust levers at 20 the aircraft will float - I haven't done a landing where it hasn't or seen a captain do one where it hasn't. If you try and fly it to the ground carrying the excess speed you will probably land overly firm and possibly bounce. Either way it probably wont be controlled. I have found closing the thrust levers later, in a headwind, works quite nicely for the same reason. I am not massively experienced so I could be a million miles off but it is what I have been taught and it seems to work for now until I gather more experience in different circumstances.
The reason I offered 10 knots as a tailwind is because this is the limit of the aircraft.