White pushes his KRPawn out to KR3(h3)
to control KKt4(g4), preventing a 6..B-KKt5
pin. Black castles short (ie. K-side), bringing his
King's Rook into play.
White Black White Black
1. P-K4 P-K4 6. P-KR3 0-0
2. B-B4 Kt-KB3
3. P-Q3 P-B3
4. Kt-KB3 B-K2
5. 0-0 P-Q3