The shikan kan is an easy to make fist, using it is the difficult part. The "extended" knuckle being referred to is that of the middle finger.
Point Of Impact:the middle joint of the middle finger
PURPOSE:to strike vulnerable pressure points
Strengths:the small striking surface of a single knuckle gives you more force to work with, pressure points are more easily breached
Drawbacks:the ammount of force being placed on the single finger is too much for it to handle if a hard surface is struck, the window for mistakes is very small
More to Come!