Endgames, solved under the rules you actually play.

Without the rule this is mate in 69. Within the rule it is mate in 84.

The fastest mate takes too long between captures, so it can't be played. To keep the clock alive, White has to take a longer route, and with a fresh clock, the longest forced mate in this endgame is 84 moves. Only one first move keeps it: Kb2.

23 legal moves. One wins.

The halfmove clock reads 82. A table that ignores the clock reports 12 winning moves. The two fastest mates, Rf4 and Re4, are mate in 25 if you ignore the rule. On this clock, both are draws.

Mate in 184. The rule makes it longer.

Without the rule this is mate in 182. Within it, White needs two more moves. The next pawn move or capture is 97 halfmoves away, so at clock 3 it's still mate in 184. At clock 4, it's a draw.

Move the clock. Watch the winning move change, then disappear.

Without the rule, Kc3 mates in 24. Within it, a higher clock leaves less room to reach the next capture or pawn move: the mate stretches, then Rd2 and Rg7 take over in turn. From clock 96, no move wins.

O-O-O+. A fork made by castling.

The rook lands on d1. It checks the king on d6 and, along the first rank the king has just left, attacks the bishop on g1. It's the only winning move, and it leads to mate in 17. Without the castling right, the same position is a draw.

The king is in the way. Castling moves it.

Black's h-pawn queens next move. The rook on a1 can't reach h1 because its own king is on e1. Castling moves the king and brings the rook to d1, covering h1, in one move. Out of 12 legal moves, it's the only one that wins. Kd2 clears the rank too, but one move slower, and only draws. Without the castling right, White can't win at all.

Three moves win. The obvious ones aren't the best.

1. Rb1+ Ka5 2. f4! Then f5, f6, f7, and the rooks come off on d8. Four pushes.

Five pushes to go. The first one is 35 moves away.

White wins, but none of the b-pawn's five pushes can be played for 69 halfmoves. From clock 32, the win is gone.