Pros let go of the handlebar and try to get rid of as much unsprung weight as possible by getting their butts off the seat.
Not to be picky, but... Unsure if Cameron said the above or if that was your paraphrase, but technically this it is an incorrect term.
"Unsprung weight" is only the weight of the wheels, tires, brakes, etc. that bear weight on the road without the benefit of suspension. So whether your weight is in the seat or on the pegs it is never "unsprung weight".
I think what was probably meant was to get the weight supported as low on the frame as possible to lower the center of gravity?