I don't know of a scenario that is an example: if either number can be played but not both, the player must play the larger one. Can someone please explain when this rule should be used.
Just to make it an easy and fast rule, a player MUST play the larger part of the roll, if he can play ONLY one part of his dice.
In the above example, if red rolls 21, he MUST play 3/1, and is not permitted to play 3/2, this is not a choice, its a mandatory move.
Player A has 14 stones on Player B's one-point, and a fifteenth stone on his 3-point.
Player B has a wall at least two points long running back from his two-point.
If player A rolls 1-2 he can move his fifteenth stone either one or two points (respectively to either the two-point or one-point), but cannot move it three (ie remove it) because not al his stones are in his Home board yet.