Juji-Uke is effective against punches or knife thrusting attacks. 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. 

From the Forward Leaning Stance (Zenkutsu-Dachi) - Right and left hands are in the chamber (by the rib cage, fist closed / palms up). Bring both hands towards the opposite thighs, left fist to right thigh, right fist to left thigh, crossing forearms (palms in / closed).  Either arm can pass in front of the other. Repeat for opposite side.

 Variation - Jodan Juji Uke - Bring both arms upward stopping just above the head.

 Remember, without safety, there can be no Jujitsu.