I'm looking for specific best practices around hand position and motion while using a thumbpick on an acoustic guitar. I haven't found any specific resources on this. I have been impressed with Troy Grady's Cracking the Code course on pick positioning and motion, and want a similar understanding for thumb picking. Here's my current understanding:
Wrist should be rotated clockwise, towards the guitar, so that the palm is angled slightly upward.
Thumb should be straight and angled approximately -20 degrees, depending on the note (if 0 degrees is a horizontal line parallel to the neck of the guitar. Another way of phrasing this is that the angle between the thumb and the neck should be about 20 degrees.
Fingers should be angled away from the neck at a steeper angle than the thumb, around -75 degrees, and should form a lightly opened fist. Fingers should be far enough away from the thumb that the thumb does not hit the index finger on the follow through of the down stroke.
Here's a picture of Mark Knopfler, and the corresponding video.
The thumb moves at an angle down and also away from the guitar. How much of the angle of attack is down and how much is away? The thumb remains straight.
The wrist rotation plays a role here as well. The more the wrist is rotated towards the guitar, the more the thumb needs to move away from the strings to escape the strings. ScottTsai has a great video on fingerstyle technique in which he explains that the wrist rotates towards the guitar as the thumb attacks in order to give a better sound. But I wonder if that's the most common wrist rotation. Wrist rotation away from the guitar would make thumb attack more of a downward motion rather than a combination of downward and away.
The thumb pick has a downward slant, and the middle to trailing edge of the pick strikes the string.
The fingers have an angle of attack that's upward and away from the guitar, but not angled as away as the thumb is. ScottTsai says he noticed that Tommy Emmanuel strikes the strings with the inner side of the fingertips (the side closest to the thumb), not the tips. He also says that the fingers should remain in a fist as much as possible for more efficient movement.
See Tommmy Emmanuel for an example as well.