A quick example:
White makes the 1
st move (to 'A' or 'B' Level) but choose 'B'.
Bh2 - Bh4
Black moves his Knight threating Whites Pawn.
Ag8 - Ah6
After White moves his pawn forward....
Bh4 - Bh5
Black's 2nd move, his Pawn tries to pass the White Pawn
in two ways. On either 'A' or 'B' Level by moving 2 squares.
Ag7 - Bg5
White's 3
rd move captures Black's Pawn
(en-passant).
Bh5 x Bg5
and moves his pawn to Bg6
OR . . . . . . . . . . . . . . . . . . . . . . . . . .
Black's 2nd move, his Pawn tries to pass the White Pawn
On 'A' Level by moving 2 squares.
Ag7 - Ag5
White's 3
rd move captures Black's Pawn
(en-passant).
Bh5 x Ag5
and moves his pawn to Ag6