Pieces in the game consists of stacks of stones (a single stone being a stack of 1). All stones in a stack belong to the same player.
Players take turns in moving one of his stacks. A stack moves exactly as far as it's high in any direction.
It is allowed, however, to split the stack in accordance with the move distance. E.g. if the piece to move is a stack of 3 and you want to move 2 squares, the top 2 stones will be taken to the destination cell and 1 stone will remain on the old position.

A stack may land on top of another stack, friend or foe, creating a larger stack (the sum of both). All stones in the resulting stack are then owned by the moving player.

The capacity of a board position is defined as the total number of adjacent positions. So, on a hexagonal board, the corners have a capacity of 3, the remaining border positions have a capacity of 4 and the rest have a capacity of 6.
When a stack's height is at or over the capacity of the position it stands on, it'll 'explode'. I.e. each neighbour receives 1 additional stone on top of the stack standing there and the exploding stack is reduced by the position's capacity.
Explosions will cascade until all stacks are below capacity.

 
HExplocus is the hexagonal version of Explocus, invented by Martin Medema around 1980.
HExplocus turns out to be a bit friendlier to the human, because the bigger board and higher capacity makes play a bit more strategic.
The name Explocus is a contraction of 'explosive' and 'Focus', the latter being a game by Sid Sackson (see http://en.wikipedia.org/wiki/Focus_(board_game) ) that provided the basic stack movement rule (stack height = move distance).

 
Please note:
When a game has ended, ZoG will display a dialog that announces the correct outcome of a game. However, after that dialog is dismissed, another extraneous dialog pops up that always calls it a stalemate.
This dialog is not a serious problem, it's just an annoyance - simply ignore it.

This implementation uses the "Axiom" Meta-game engine.