Jodan -Uke is effective against punches or kicks to the head. The following key points should be remembered:

•  Speed is more desirable than power. All the power in the world wont help your block of you don't react fast enough. Try to redirect the strike rather than stop it.

• Never over-extend a block. Reaching too far with a strike will waste energy and make you vulnerable to secondary strikes.

• Move your body. Don't stand still when blocking. use angles to increase the effectiveness of the block.

• Always exhale when blocking. This increases power to the technique and helps protect you by contracting the abdominal muscles.

• Execute the block in such a way that contact is closer to the elbow than the wrist. 

Practice blocking techniques in front of a mirror. This way you can see what you are doing.  I carry this over into my dojo and when working with students, I will perform the mirror image of what I am teaching. 

Starting in Kibi-Dachi (horse stance). Right hand is in the chamber (by the rib cage, fist closed / palm up), left arm is bent (almost 90 degrees) in front of and above the head (fist closed / palm in).  Turning the right palm outward, bring the bent right arm upward crossing the chest, stopping above the head, while bringing the left arm, down into the chamber.  The right arm passes in front of the left arm. Repeat for opposite side. 

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 Remember, without safety, there can be no Jujitsu.