#1
If I would play this:

-
-
7
x
5
-

... Then I would fret the A-string with my index and the G-string with the ring finger, and then hitting the A with the pick while simultaneously hitting the G-string with the ring finger of my picking hand, thus not hitting the "middle" string at all. This, however, seems incredibly quirky. What've I missed? How are you supposed to play these things?
#2
fret the a string with your index finger, and arch it only slightly so that it covers all the strings below it, but doesn't allow them to ring out... thats how I do it, anyway
#3
Just mute the middle string (in this case the D string) with your middle finger, that way you can strum without having to worry so much about your right hand.
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Fender Am. Standard Stratocaster
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E-Bow
#4
Well it's basically a D5 chord, just take off your middle finger slightly to mute the string, like Johnius said
#5
I wouldn't use my middle finger. Your first finger holding the A string should lie flat over the lower (thinner, that is) strings, dampening them, while at the same time slightly touching the higher (thick) E with your fingertip, dampening that. With this you should be able to strum all the strings (like a normal chord) but only the one string will ring out. Then just add the octave note with your ring finger, and strum away. What you described is "hybrid picking", which is a legitimate technique and has it's place, but probably not in this instance.
#6
Lay your index on the A string flat enough so it deadens out the D string, then put your ring on the G string.
#7
I tried muting the D by arching my index. Works quite well. Thanks, everyone!