A forcing move is a move that makes a threat that your opponent must reply to on their next turn. Common forcing moves include playing in one of the empty hexes of a bridge, intrusion into an edge template, or threatening to complete an immediate strong connection or win. Consider the following position with Red to move.
At first glance, the position looks bad for Red, but she can win by making a couple of forcing moves. She plays at e8, threatening to play at e7 on his next turn which would create a winning connection. Blue has little choice but to stop this threat by playing e7, since there are no better options. The move e8 is a forcing move.
The forcing nature of the move allows Red to place a piece on the other side of Blue's line without giving Blue time to do anything constructive. The e8 piece on the other side is connected to the bottom and is extremely useful to Red.
Red continues by playing another forcing move at g7. The only move that stops this piece from immediately connecting to the bottom edge is f9. But after f9, Red completes the win by playing at f8.
(Note: the two forcing moves could just as easily be played in the reverse order. That is Red plays g7, Blue is forced to respond with f9, and then Red plays e8 which threatens to form an connection in two distinct ways.)
In general terms, the opponent has three options when responding to a forcing move.
- Defend against the threat, by saving the connection, restoring the template, etc.;
- Ignore the threat and play elsewhere (see also tenuki);
- Counter the threat by making a forcing move of their own.
A player who attempts a forcing move usually expects the opponent to defend. However, if the threat is not strong enough, the opponent may decide to do something unexpected, such as ignoring or countering the threat, or even defending it but not in the expected way. Therefore, care must be taken to ensure that the potential forcing move really forces the intended response.
Some forcing moves are better played early than late. For example, with very few pieces on the board, the opponent might typically answer a bridge intrusion, because they expect that the bridge may become important later in the game. Later in the game, it may turn out that the bridge is no longer important to the opponent's connection, and the opponent would likely not defend it at that point.
Breaking edge templates via forcing moves
Forcing moves are the only way to successfully defeat a template. This is done by making a template intrusion that is also a more threatening forcing move. After the opponent responds to the greater threat, the attacker can play another move within the template and destroy the connection. For example, consider the following position with Red to move.
The piece on g3 is connected to the right edge by a ziggurat, indicated by the "*"s. If Red moves at h2, she intrudes on the ziggurat while also threatening to connect via g2. Blue can only stop this threat by playing at g2. Then Red Plays i3, breaking Blue's connection to the right.
Using forcing moves to steal territory
By playing a forcing move inside the opponent's template, a player can sometimes steal territory at no cost.
In this position, if Red intrudes on the left side of the bridge and Blue defends the connection, Red gains territory on the left, while giving up nothing of value on the right, and without disturbing either player's connections.
Significantly, the additional territory is on the other side of Blue's connection where it may potentially be used for a future threat. The additional territory can't hurt and sometimes makes a crucial difference.
A forcing move is harmless if it gains no territory for the opponent. You should not be worried at all about leaving harmless forcing moves available for your opponent.
When you have more than one way of completing a connection, e.g. when completing a loose connection, you should consider which move leaves the least valuable forcing move for your opponent. Consider the following position with Red to play.
Red wants to connect her two pieces. There are three distinct moves that accomplish this, d3, c4 (bridging to d2), and c3 (bridging to d4).
There is not much to be said about d3; it directly connects without altering anything else.
c4 connects, but gives a potentially useful forcing move to Blue. Blue can respond with c3. If Red saves the connection with d3, Blue has gained a free hex of territory. The hex c2 is now directly adjacent to the c3/b4/b5 group when it wasn't previously. Hence, c4 is worse than d3.
Now consider the last remaining possibility, c3. This leaves two forcing moves for Blue, but both of them are completely harmless! If after c3, Blue plays one of the forcing moves c4 or d3, then Red can save the link and Blue will not have gained any territory at all — any empty hexes adjacent to the forcing piece were already adjacent to Blue's existing pieces. Hence, c3 is just as safe as d3 but significantly, c3 gains one hex! — b3 is now adjacent to Red's d2/b3 group when it wasn't before. Thus, c3 is better than d3 and is the best of three choices. In other words, c3 dominates c4 and d3.