The way I understand it is that ranks 5-8 are shifted one square left:
h5
h5'
a4'
a4
Whenever a move passes through a4 from below, it can be continued normally, or like the upper board half was not shifted (i.e. from a4'), and similar for h4 reached from above. a4' cannot be reached from below.
The way I understand it is that ranks 5-8 are shifted one square left:
Whenever a move passes through a4 from below, it can be continued normally, or like the upper board half was not shifted (i.e. from a4'), and similar for h4 reached from above. a4' cannot be reached from below.