More famously known as a "Karate Chop," the Soo Doh is an open hand strike made with the medial side of the hand (edge of palm/hand next to the little finger). Fingers are held tightly together with the thumb locked against the lateral side of the palm. Although a "flat hand" position, the metacarpals should be slightly rounded (think of the hand position made when splashing water on your face, but not as much as when cupping the hands to hold water). This prevents damage to these bones when your hand makes impact with its target.
Typical execution cocks the hand back near the ear on the same side if performing the strike vertically, or horizontally with the palm up. If performed horizontally with the palm facing down, the hand is cocked to the opposite ear instead. Usual targets include soft tissues and other sensitive pressure points, although with proper conditioning, this strike can easily break the long bones in the body (particularly effective against the clavicle).