From Sensei Pat Moriarty Pine Forest Karate Dojo
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Wrist Position

We often hear the correction to bend the wrist/fist
forward when performing Jodan Uke, (high block). Just how much is enough?
Bending the wrist forward too much can be detrimental to the effectiveness
of the technique, similar to the like of taking too many aspirin. Since
all bodies are different, the karateka can best determine how much of a
bend is enough for them. One method of checking the proper bend of the
wrist is by placing the first two knuckles of the fist on the makiwara as
if punching. Make sure that the arm is fully extended, sloping slightly
downward and centered (as if striking someone your size and stature in the
solar plexus). The strike should result in an exact skeletal muscular
alignment from the shoulder to the first two knuckles of the fist thus
allowing the devastating forces created by the entire body to be
transmitted to the target. Observe the wrist position necessary when
obtaining this alignment. This is the position the wrist should be in when
performing tsuki waza, jodan uke, chudan uke, gedan uke, hiji ate waza, and
many more techniques…. In fact, one should observe this wrist position
throughout the full travel of the technique. This is the position that
will allow you to maximize the tension of the muscles in the forearm and
result in a formidable technique.

Another method of determining the proper wrist position, as described by
Hanshi, is to hold a knuckle push-up position with the arms in line with
the fists directly under the shoulders, the palm side of the hand/fist
facing towards the feet. Then lean on the first two knuckles (which should
take the brunt of the force of the strike). In this way, one gets the
correct wrist angle in four ways/direction, front, back, right and left.

Arigato
Pat Moriarty
Pine Forest Karate Dojo