31. A Rook cannot perpetually chase a Pawn that has crossed the river. It doesn't matter if the Pawn also threaten to capture a piece. The Rook's side has to change or lose.

Diagram 74: Red (in Capital) moves first

R45 p56
R54 p65
R45 p54
R56 p45
R65 ....

Explanation:
The Red Rook's protecting Red Knight cannot justify perpetually chasing the Black Pawn. Red is violating the rule and has to change.


Diagram 75: Red (in Capital) moves first

R65 p54
R56 P45
R65 P54
R56 P45
R65 ....

Explanation:
The Black Pawn's perpetually chasing the Red Cannon does not violate the rule, the Red Rook's perpetually chasing the Black Pawn does. Red is violating the rule and has to change.