Vocabulary: Ryu=promoted Rook, wa=as for, soto=outside, ...yori=rather than ..., uchi=inside
The promoted Rook is most destructive when it has penetrated deep inside the opponent's camp.
Every shogi piece is meant to move forward, so an attack from the front can be coped with more easily than one from the side. Therefore, the promoted Rook can be quite effective as a tool to threaten from the side.
White in hand: N L P2 9 8 7 6 5 4 3 2 1 +---------------------------+ |wL wN * * * * * bR+ * |a | * wK wG wS * * * * * |b | * wS wB wG * * * * * |c |wP wP wP wP wP * wP * wP |d | * * * * * * * * * |e |bP bP bP bP bP * bP * bP |f | * bS bB bG * * * * * |g | * bK bG bS * * * * * |h |bL bN * * * * * * wR+|i +---------------------------+ Black in hand: N L P2 Diagram 1.Diagram 1 shows where each player has promoted his Rook. What the proverb teaches is that the promoted Rook deeper within the opponent's territory is better. The reason is...
P*2i.
Black can drop a Pawn on 2i thereby blocking the promoted Rook's path. White will then play +R1h, but in this case, the promoted Rook would do a much better job in the bottom rank.
Now look at Diagram 2.
White in hand: N L P 9 8 7 6 5 4 3 2 1 +---------------------------+ |wL wN * * * wP * bR+ * |a | * wK wG wS * * * * * |b | * wS wB wG * * * * * |c |wP wP wP wP wP * wP * wP |d | * * * * * * * * * |e |bP bP bP bP bP * bP * bP |f | * bS bB bG * * * * * |g | * bK bG bS * * * * * |h |bL bN * * * wR+ * * * |i +---------------------------+ Black in hand: N L P2 Diagram 2.In the above Diagram, White's promoted Rook is even closer, on 4i. It will be your turn to face difficulty when White drops a Pawn on 4a.
Generally speaking, the big pieces like Rooks and Bishops function better when placed at a distance and aim at the opponent's King from afar, but when both you and your opponent have the promoted Rook in each other's camp, it would help you if you could place yours closer to the opponent's castle.
Remember, the promoted Rook is also good for defense.