He is using ISO 320 film and exposing it as if it were ISO 160. This is over-exposing the film by 1 stop. He's then under-developing the film by 20% off the normal development times recommended for shooting at ISO 320.
This technique is called Pull processing. Its reverse is called Push Processing.
In Pulling film, you overexpose the film and then reduce development time to compensate for that overexposure. In Pushing, things are reversed - you underexpose the film and then overdevelop to compensate.
There are recommended developing charts for just about every modern film and developer combo, pushed and pulled. Massive Dev Chart.
That being said, you really need to test things out to see if you like the result. In your case, Nixon really likes the shadow details of Tri-X pulled 1 stop and then developed at normal minus 20%. Will you like the combo? Maybe...maybe not.
Keep in mind the format as well as this changes things a bit. For example, I really like shooting Delta3200 at ISO 800 (pulled 2 stops) and developed in Rodinal 1+100 for 60 minutes - when shot on 120 film. However, the grain is just too much when using this combo with 35mm.
Do some more reading on Pull and Push processing and give it a go!