Black to move
This is a position from one of the games I refereed in the 1993 World Championship. White has just played h4.
Black to move
Black had quite a long think at this point, so I did too. Putting myself in Black's shoes, I am worried about White going to h6 on his next move. Why am I worried? Well, I am a bit worried about my position anyway, as I haven't got many good moves, thanks to all those black discs on the periphery. Let's go round the board and see what there is. a2 is a C square, and anyway going there neutralises White's weak west edge with its disc on a7. a3 would be answered by White's semi-forced move a2, and also removes his weak edge. f8 flips the four discs above it and takes away my moves in the lower part of the g column. g8 is another C square. g7 or g2 would immediately give White a corner. What about g5 or g6? Either of these gives White access to the other move of this pair, an area where he currently has no moves. They also make the black periphery even more extensive. h2 is another C square and a particularly bad one as White would be able to wedge at h3 and then get the h1 corner. Going to f1 is worth considering, but makes my position worse in the North. Also I am worried about a possible White move to h6: what would I do next? My last available move h3 stops White going to h6, but it flips g4 and f5 and thereby allows White access to g5 and g6, giving him yet more moves. Also it establishes me on an edge, and maybe I don't want to take any edges yet.
However, if I don't go to h3 but go somewhere else instead White will go to h6 and then I will have to find another move in an even worse position than I am in now. If only there was another way of stopping White going to h6!
Well, there isn't, however hard you look. But what Black can do is make White's h6 move rather less attractive for him. The move that Black found to do this was g6.
After g6
Notice that this links the h6 square with the rest of the board, so if White goes there the five discs on c6 through g6 will be flipped back to white, giving Black access to c7. So Black ends up with an extra move which he didn't have before (Black might play g5 first), and one extra move is sometimes all you need to survive.
I hope this has illustrated a tactic you should always try to bear in mind: if you can't stop your opponent making a move you don't want him/her to make, don't give up and resign yourself to your fate; instead, look for some way of making the move unpleasant for him/her. This is called poisoning a move. Quite often the poison is much more deadly than that used in this game.