Copyright (c) 2001 W. D. Troyka

Sabin Rains is played on an empty 5x5 hexagonal board:

HEX RING - The set of six cells surrounding any cell on the board
DROP - On each turn, each player drops a stone on an empty cell.
A stone cannot be placed on a cell which does not have an adjacent empty cell.
When a stone is placed, it flips all enemy stones that are sandwiched between itself and a friendly stone along a hex ring. 
When five enemy stones are located on a hex ring, the dropped stone will flip all five.
GOAL - When no more moves are available, wins the player with more stones.

An example

Red's move. He drops a stone at cell [1] capturing only the marked blue stone. Now, Blue can only drop on one of the green dots. After any one of those moves, Red has a winning reply.

Some notes from the author: Be guided by three considerations. First and most obvious, you want to win the battles for individual hex rings by placing the sixth piece, which typically results in the entire ring being flipped to your color. It is common that the sixth piece placed in one hex ring is the fifth piece for another, so that your opponent may complete a hex ring on the next move. You should try to look ahead so that you will place the last sixth piece when a series of hex rings is being flipped. Second and more important, you want to lock in your gains in those portions of the board that are no longer flippable. `Locking in` occurs when a portion of the board closes in on itself and no further moves are possible in that area. Ordinarily you can lock in the most pieces by playing the last piece in a board area before it closes in. Finally, your overall strategy should be to play the last piece in as many closed-in areas of the board as possible, which will typically maximize your gains.

There is a ZRF to play Sabin Rains with Zillions.