As you can see from my video below. I am simply using an inversion to play the 9th key with my 4th finger instead of trying to reach across the keyboard for the 9th in it’s natural place or using two hands. This is a very useful if you need to play melodies with 9th chords at the same time. Also check out the diagram below.
The first three chords I am playing by the way are Bbmin9th, Gmin9th then Fmin9th.