Bass legend Carol Kaye writes that she uses a "flat wrist" technique when playing bass with a pick.
What does she mean by this?
It means to not rotate your hand inwards towards your inner arm. If you look up videos of Eddie van Halen playing some of his solo's, esp. when he's rapidly picking. He often curves his wrist over in a manner that is inappropriate for bass (according to C. Kaye). Here's a good negative example, with primarily rotational motion (note: I'm only trying to indicate that C. Kaye would frown on this kind of right hand technique for playing bass). Or here is a longer discussion -- note the emphasis on the rotational motion.
Stick your arm straight out in front of you, palm down, but parallel to the floor. Now drop your hand at the wrist -- C. Kaye advocates for avoiding this position/motion when playing bass.
Stick your arm straight out in front of you, palm down, but parallel to the floor. Now rotate like you're turning a doorknob. -- She'd say to avoid this kind of motion in wrist as well.
Stick your arm straight out in front of you, palm down, but parallel to the floor. Now rotate your hand left and right at the wrist while keeping the palm parallel to the floor. This is the primary wrist motion that C. Kaye advocated; she believes that the wrist is in a sense strongest in direction of motion, and allowing other aspects of motion (curving inward or rotating the forearm) are liable to lead to injury.
Here's a reasonable example of her playing -- note that the primary motion is as I've described, although there are a few points where she uses some fore-arm rotation type of motion to get extra pull on some of the notes, particularly up-strokes..