Contrary to what a few folks may be teaching, the flat left wrist (for right-handed golfers) is a must. You can bend your wrist (hold hand out straight in front of you and bend fingers towards the sky or the ground) and still hit the ball, but you’re adding another plane to the swing making it that much more difficult to be consistent.
Players that mistakenly bend their wrists as I’ve described above, usually do it during the first 18 inches of their backswing. To correctly initiate the backswing use your torso – think shoulder rotation. Your hands should just be along for the ride. There is a tendency for many golfers to use their hands immediately from their setup position to start the golf club into the backswing and this causes the undesirable wrist break.
There are a lot of drills out there for correcting this problem, such as using popsicle sticks, but the easiest solution is just to focus on the takeaway. You’ll quickly ingrain the feeling of keeping your hands still.
You can keep your left wrist flat and still hinge your wrists for power, but many golfers don’t correctly understand how to do this. If you’re a right-handed golfer, hold your left arm straight out in front of you. Rotate the wrist so the back of your hand faces the target. Now move your thumb so it points to the sky. This is the wrist break you’re after.
If you keep you left wrist flat you’re well on your way to becoming a much more consistent ball striker.