The wrist of the dominant hand is the most important joint in shooting a handgun. It can bend upward or downward, inward or outward. It also tends to go to its strongest position as the muscles of the forearm contract.
The strongest position of the wrist is often referred to as its neutral position in that with equal contraction of the forearm muscles, the wrist will be equal or "balanced".
If the sight picture is adjusted by motion of the wrist, the wrist will most likely return to its strongest or neutral position as the trigger is pressed, particularly if the shooter is anticipating recoil. This will likely direct the shot away from the original point of aim.
For this reason, the handgun should be grasped firmly with the wrist in its neural position. Adjustments in sight picture should be made by movement of the feet and arms.
One of the dogmas of handgun shooting is that the barrel of the gun should align with the bones of the forearm when the gun is grasped in the firing position. Whether it does so will depend primarily on the relationship of the size and shape of the gun to the anatomy of the shooter.
When the gun is grasped with two hands, the alignment of the barrel with the bones of the forearm will make the gun point off-center, toward the non-gun-hand side. This will require a few to as many as 30 degrees of bend in the wrist to get the gun pointed at the target.
When you grasp the gun with both hands you provide a balance of strength between both hands and thus return the effective position to one of neutral or balance, so the shot will be more likely to hit your target. If the pressure provided by both hands is not equal, you will tend to shoot toward the weaker grip.
We consistently tell students to grip the firearm firmly. What is firmly? The easiest way to describe it is in terms of the "white knuckle express". I think most have seen someone that is frightened of their current situation (such as flying) and grasp the chair arm so tightly that their knuckles turn white. That grip on a gun would be far too tight.
Grasp your forearm with your dominant hand and look closely at your fingernails. Squeeze tighter until you notice the blood start to go out of the fingernail, emphasizing STARTS. THAT is how tight you want to grasp the gun. Any tighter and you begin to tremble. Any looser and you do not have sufficient control of the gun to shoot effectively.