The counterpart of Saju Jirugi, built around defence. Each direction is covered twice: first with a knife-hand low block while stepping back, then with a middle side inner-forearm block while stepping in. It is likewise practised to both sides.
The knife-hand low block cuts a low kick with a narrower, more penetrating surface than the forearm.
The inner forearm block deflects a middle attack outward, opening the opponent's flank.
The left-hand column shows where you face and which stance you form, seen from above; the red foot carries the greater weight. See the direction chart in the Encyclopedia if you cannot remember where each letter falls.
Summarised from Choi Hong Hi, Encyclopedia of Taekwon-Do, volume VIII. The order of movements and the diagram follow the book; the explanatory texts and the footwork schematics are our own work.