STOP 
You're going about this wrong. You have been taught how to perform hundreds of physical actions in your lifetime with neither descriptions of muscles nor modes and planes of movement.
This next part is not technique. This is so you can see what's going on. Hold your strong hand out in front of you, thumb-side of your hand upward, but hand and arm relaxed. This is the neutral hand position. Now, keeping that arm and hand relaxed, take your weak hand and move the strong hand up and down at the wrist the way the gun would move under recoil. Again, no fist. Relax the hand and arm. Use the weak hand to move your strong hand up and down. NOW make a hard fist with your strong hand and try the same thing. Wrist doesn't move? That's the lesson. Of course, YOU can CHOOSE to move the hand up and down with a hard fist, but it's no longer a soft wrist. The act of making a fist tightens that wrist into place. Hard fist = hard wrist. If you like, you can do the same thing and start with a relaxed fist, letting the weak hand move that wrist up and down, then slowly tighten the strong hand fist so that you can feel the wrist tighten up.
That's how you lock down the wrist -- make a fist. Or in terms of shooting, GRIP your pistol hard. You don't lock the wrist in during shooting by locking the wrist. You lock the wrist by gripping the pistol hard. There are a couple of acronyms for this that are inappropriate for this family-friendly forum, but I'll give you the G-Rated version -- G.T. BOOI --
Grip The Bejeezus Out Of It. Or the R-rated versions -- G.T. SOOI & G.T. FOOI -- I'll let you figure out what those alternate letters stand for.
The second part of the steel grip is accomplished at the elbows (this is a lie; it's at the shoulders, but the easiest way to conceptualize it is with happening at the elbows
). Holding the gun out in front of you, hard grip, rotate your elbows out. This will clamp your hands down on the pistol. If you don't have a pistol handy, clasp your hands together, fingers inter-laced, and hold them out in front of you, or grab something small to grip like a gun, and present it or your clasped hands like you're aiming at a target, preparing to fire. Now rotate your elbows out. Can you feel your hands clamping down on the pistol? You can get a tremendous amount of additional grip force out of your shoulders by rotating your elbows out.
So improve your grip strength. Your shoulders are already strong enough to tear your grip apart, so no need to improve there, unless... you know... fitness?
But every bit of additional grip strength you earn will also allow you to clamp down even more with stronger pronation from the shoulder. Sorry!! -- I mean by rotating your elbows out harder.
This is the easiest way to conceptualize it, and conceptualizing counts for a lot.
Also, a youtube search for "Bob Vogel on Grip" will produce a pretty good video. It's not quite the same with a CZ as with his Glock, but the fundamental principles are the same.
