I think it's just a matter of practice. It is possible to keep your wrist firm but your grip somewhat relaxed, but I agree it's difficult. The arm and hand has something like 40+ muscles, so you got to learn to contract the very specific right ones and let the others relax.
From my understanding of a cupped wrist/cranker release, I've always believed there was two ways to do it.
1. The wrist is cupped at setup and remains cupped throughout the entire approach until release where here the wrist uncups in a powerful snap creating the high revolutions cranker players get.
2. The wrist is just firm, maybe even open from setup to the top of the back swing, but on the start of the downswing the player will cup his wrist and keep it cupped till that release point where it snaps and uncups as in #1.
Keep in mind many crankers also utilize a slightly bent elbow on the downswing to better stay under the ball and gain leverage, that's something that might be useful to you, but it does require strong conditioning of the arm to avoid injury.
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