Gedan-Uke is effective against punches or kicks to the groin. 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 extended in front of the left outer thigh, (fist closed / palm in).  Bring the left hand in front of the groin while moving the right hand in front of the left shoulder (fist closed, palm facing chest). Bring the right hand downward so that it crosses the body, stopping when it is just outside of the right thigh. At the same time, withdraw the left arm into the left chamber. Repeat for opposite side.  

1    2 

3    4  

 Remember, without safety, there can be no Jujitsu.