This would be "muting" the strings, and you simply lay your hand flat against the strings. The finger pressure should be enough to stop the noise of the strings and produce the "chicka chicka" when the pick hits the strings, but not pressed too hard as to produce a noise of the frets.

When you are performing this you should first concentrate on keeping the rhythm hand going NO MATTER WHAT during the rhythm playing. This will definitely improve your timing and it will help make sure that when you land the chords that they sound rock solid. This actually makes the muting (chicka chicka) EASIER once you get used to it.

As far as landing the power-chords from muting and back, it simply requires practice, but your fingers are not moving far, they are simply relaxing against the strings, so having them curl back up to grab the chord is not a far stretch of movement and should be easy to get down with a little practice.

Hope this Helps.
James

www.jamesericksonmusic.com