So how'd I play this?

[daveyh]

If it's metally the muted string should add in a nice percussive 'chuck'

[erictheweary]

If it's metally the muted string should add in a nice percussive 'chuck'

It is a damped/muted string rather than an unplayed one so if you strum it it creates a noise. Not one you are likely to be able to pick out in a band mix probably but playing octaves in isolation you would hear something. If you read stuff about Wes Momtgomery or Jimi Hendrix ( the most obvious populisers of using octaves melodically on guitar) that middle string tump is usually decribed as part of their sound.