Copyright (c) 2003 Mark Steere

Impasse is played on a 8x8 square board with the following setup:

Black lies in the bottom, green in the top of the board. 
SOLDIER - A soldier slides diagonally forward any number of empty cells.
KING - A King slides diagonally backwards any number of empty cells.
TURN - On each turn, each player must do one of the following actions:
Move a friendly piece
Swap a friendly King with a friendly soldier, provided that they are adjacent and the King is in front of the soldier.
IMPASSE - If a player does not have a valid move, he must do one of the following things: (i) remove one of his soldiers from the board, or (ii) demote one of his Kings to a soldier.
After the impasse, it is the next player's turn. So, each player can only apply the impasse rule once for each turn.
This rule only applies at the beginning of the turn.  If a player makes a move which deprives him of future moves, this does not entitle him to the impasse.
PROMOTION - If a friendly soldier moves into the player's last row, it will be promoted to a King and the player must remove another friendly soldier from the board (using checkers, this means that the removed soldier will be on top of the promoted one).
If there are two soldiers in the last row (yes, it's possible), the player must immediately promote one and remove the other. 
If there is one soldier in the last row, and no other soldiers on the board, then the soldier remains uncrowned.
DEMOTION - If a friendly King moves into the player's first row, it will be demoted to a soldier.
GOAL -.Wins the player that removes all, or all but one of his pieces.
A promoting example

Black can promote the bottom soldier sliding it to cell [1] and thus removing another black soldier.


Black's turn. Since he cannot move, the impasse rule is applied and Black may remove one soldier (or demote one of his Kings). The best option is the bottom soldier, because it provides the best chance of keeping the impasse on the next turn. 

More information can be found here.