I don’t know if this has been figured out on the site, but I thought I’d put my progress on working out Martin Miller’s finger movement. I think the issue isn’t with the index finger, it’s the failure of the thumb to follow the index finger.
So what I did, having tried moving my index finger up and down quickly was to try the same with my thumb. The thumb hardly moves, the index finger moves brilliantly.
So, I thought, as I watched the grass court tennis, what if I tremolo my thumb with the lifting movement in the air. Did not work. Thumb would not move.
So, what if I used the index finger of my fretting hand to rest against the side of the thumb while I tried to tremolo the lifting. Slowly, over four days, it released. I then put my picking hand finger and thumb together and used the index finger of my fretting hand sticking out straight on the knuckle of my picking hand index finger to measure if I was lifting the thumb and finger together quickly. I am. Nice.
What I noticed when I first tried to moved my thumb quickly is that it sets off my forearm rotation. I cannot play this without forearm rotation. This is less so the case with the index finger tremolo on its own.
I then spent about a week synchronising in the air the rotation and the finger movement.
And then I picked up the guitar and a plectrum, and I’ve got to the process now of seeing if my joints are the right ones to turn the air guitar into real guitar.
My first attempt produced a swooping on both sides of the picking fingers, which slows down the picking speed, which is why Martin seems to use the sweeping only to move rightwards, but the tone of the notes is very good.
So I can see some experimentation will be in order.
I’ve come to the conclusion it is very hard to learn this while sitting with a guitar in your hands rather than in the air without the guitar. Not the conclusion I expected.