Ude uke (literally: "arm block") is essentially an inside version of the cross-body block, as it also redirects attacks above the diaphragm to the side. Within Goshin-Jutsu, ude uke is sometimes referred to as a picture-window block (...although no one is quite sure why).
Start by raising your fist to head level, and raise your elbow to solar-plexus level, one fist-width away from your chest. Other karate styles say that the elbow should be raised to the side at head level, but Goshin-Jutsu does not, in order to protect the floating ribs. Complete the block by twisting at the waist to pull the forearm across your chest, so that the blocking elbow is in front of the opposite side hip. The opposite side hand pulls to the hip in reciprocal action. The motion is similar to an inside hammerfist block, or a tate-shutō uchi.
[Video of ude uke, fast and slow, from the front and from the side]
When performing ude uke, it is important to focus on the attacker, and not the block; concentrating on the block leads to hip over-rotation, and this excessive motion will slow down your counterattack.