Uraken is the name of the backfist strike. The illustration shows the impact area of the fist when Uraken is delivered, the proximal side of the first knuckles of the index and middle fingers.
Seiuchin Kata and Naihanchi Kata both include this strike. Here the fist is chambered at the side of tori's head with the elbow pointing toward the target/uki. The wrist is cocked and firm, ready for impact (uki may be moving so you want the weapon to be prepared at all point along its trajectory). The trajectory is straight forward toward the bridge of uki's nose. The return is scraping downward and back to tori, palm remains horizontal throughout.