So it looks like AO is tracing the the number ‘8’ when switching strings. That’s a good visual cue. I never considered this.
I think the concept of going ‘straight up and straight down’ for the first 2 notes on the D string and then switching to DSX before playing the 3rd note is for efficiency. It is a larger ‘pivot’ to go from primary DSX to USX when picking the 4th note on the G string as opposed to just slightly tilting your wrist as required from the ‘neutral position’ of ‘straight up and straight down’ for the first 2 notes on D String, ‘tilting’ to DSX for the 3rd note on D string and then USX for the 4 note on G String.
But isn’t this just TWPS as Troy mentioned? Except the difference being that AO may be doing it with a bit more ‘efficiency’ by not being primary DSX for the first 3 notes on the D string?