Hand position requires wrist to bend, striking with outside of bent wrist pointing back toward your own elbow, or the inside of wrist bones with the palm pulled back for an inside strike.
Point of impact is the bony protrusion of the arm bone on the thumb side of the wrist joint. It can be applied like furi uchi, but is most often used striking upward (to chin), forward (to groin) or horizontally (to ribs, solar plexus, groin, or side of face).
It can be practiced like the reverse ridge hand, coming from opposite hip angled upward toward target, it can also be used to strike with palm up.
Or you can practice the basic motions of both inward and outward wrist strikes as demonstrated here.
As usual, power comes from body movement (coordinating forward weight shift and hip rotation).