The 7 flat 9 chord is a useful variant on a straight dominant 7th chord when
you're in a minor key.[ E7 in the key of A minor ]
In this example I'm playing an E7b9 at the open position then going to an inversion of the same chord before resolving on an A minor.
To get the inversion you simply move the shape ( shown below ) up three frets - leaving the open strings open.
This works like a diminished chord in that you could keep going up three frets at a time and it is still the same chord as long as you have the open E note at the bottom.