If you position your finger correctly, you can press G and mute A with the same finger, while letting the other strings ring. This way you can strum really hard and do stuff that you'd normally do only with open string positions. You can also do percussive stuff when muting all or many strings.
How do you mute a string and play another at the same time?
You mute with your fretting hand. A way to mute a single string with your fretting hand is to place one finger on the string as if you are going to fret it, but don't press with the finger to fret the note. Just keep the finger gently touching the string.