The normal chess rules are valid, with the following extensions:
Rule #1: Whenever a piece is captured, an explosion occurs on the regarding field. The explosion destroys:
capturing piece
the captured piece
all pieces on the immediate horizontal, vertical and diagonal neighbour fields, except pawns.
Destroyed pieces are removed from the board.
After moves 1. Nf3 f6 2. Nd4 Nh6 3. e3 c6 4. Nf5 we reach this diagram:
Diagram 1.
 FEN: rnbqkb1r/pp1pp1pp/2p2p1n/5N2/8/4P3/PPPP1PPP/RNBQKB1R |
Black decides to capture the knight with 4... Nxf5
According to Rule #1 an explosion occurs and we reach the following diagram:
Diagram 2.
 FEN: rnbqkb1r/pp1pp1pp/2p2p2/8/8/4P3/PPPP1PPP/RNBQKB1R |
As you can see, the explosion removed both pieces but left the f6-pawn, all according to Rule #1
Rule #2: A game is won by checkmate or causig explosion of the opponents king by an indirect hit. Checkmates can win a game only if the oponent can't explode your king move after checkmate<
Rule #3: Moves causing own king's explosion are not allowed.
NOTES:
This means that the King may never capture a hostile piece.
Also it may lead to the weird possibility that a king moving into reach of opponents king cannot be captured directly, since the opponents king would be destroyed. Still the game can be won by an indirect hit, capturing an opponents piece near his king and away from the other.
After moves 1. e4 e6 2. d4 Nh6 3. g4? Qh4 4. Kd2 Bb4+ 5. Kd3 Nc6 6. d5 Ne5+ we reach this diagram:
Diagram 3.
 FEN: r1b1k2r/pppp1ppp/4p2n/3Pn3/1b2P1Pq/3K4/PPP2P1P/RNBQ1BNR |
Here black threatens to win the king with the knight, and if white king moves black can explode the pawns near the king
So, only move for white is to remove the knight: 7. dxe6 and we reach this diagram:
Diagram 4.
 FEN: r1b1k2r/pppp1ppp/7n/8/1b2P1Pq/3K4/PPP2P1P/RNBQ1BNR |
The game continued: 7... Qf6! 8. e5 Qa6 9. Ke4 Qd3:
Diagram 5.
 FEN: r1b1k2r/pppp1ppp/7n/4P3/1b2K1P1/3q4/PPP2P1P/RNBQ1BNR |
Because of Rules #3 white can't take the Queen because then white's own king will explode. So white has only one move left: 10. Kf4
Diagram 6.
 FEN: r1b1k2r/pppp1ppp/7n/4P3/1b3KP1/3q4/PPP2P1P/RNBQ1BNR |
Can you guess the final move here?
For more experineced players I've prepared the Advanced Rules page