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 open hexes in a two-chain (threatening to break the link), intrusion into an edge template, or threatening an immediate strong connection or win. Consider the following position with the vertical player to move.
At first glance, the position looks bad for Red, but Red can win by making a couple of forcing moves. He plays at e8 threatening to play at e7 on his next turn which would create an unbeatable winning chain. Blue has little choice but to stop this threat by playing e7 (there is nothing better). 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 any time to do anything constructive. The e8 piece on the other side is connected to the bottom and is of critical importance.
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.
The group of red pieces near the bottom are connected to the bottom edge. These pieces are connected via two-chains to the group g3-g4-f5 which is in turn connected to the top edge via edge template IIIa.
(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 unbeatable chain in two distinct ways.)
In general terms, you have three options when responding to a forcing move in a two-chain.
- Save the link by playing the other move in the two-chain.
- Play elsewhere (e.g. playing another move may give another way of meeting the threat thus rendering it harmless)
- Respond with a forcing move of your own.
Breaking edge templates via forcing moves
Forcing moves are also the only way to successfully defeat an edge 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, you can play another move within the template and destroy the connection to the edge. For example, consider the following position with the vertical player to move.
The piece on g3 is connected to the right edge via template IIIa indicated by the '*'s. Red's best move is to play at h2. This intrudes on the edge template, is connected to the top via edge template II, and threatens to complete an unbeatable chain by playing at g2 next turn. Blue can stop this threat only by playing at g2. Then Red Plays i3 breaking Blue's connection to the right.
Using forcing moves to steal territory
In this position, Blue has two more hexes of territory than Red (9 vs. 7 adjacent hexes). Suppose Red makes the forcing move at the indicated hex and Blue saves the link.
Now Red has two more hexes of territory; i.e. Red has stolen 4 hexes of territory 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 it 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 with a two-bridge, e.g. when completing the loose connection described previously, you should consider which forcing move is least valuable for your opponent. Consider the following position with Red to play.
Red wants to connect the d4 piece to the d2 piece. There are three distinct moves that accomplish this, d3, c4 (two-chaining to d2), and c3 (two-chaining 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 and suppose Red saves the connection with d3. Now 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.