One positional characteristic that indicates you can do this is when the center is blocked. By playing ...h6, ...g5 you are beginning a flank attack. it is almost never a good idea to initiate a flank attack when the center is open or fluid. The principle is flank attacks are best thwarted by counterattacking in the center.
When can black play h6, g5 to thwart off a pinning bishop?
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The principle that I described is inextricably intertwined with the opening that you are playing and the position on the board.
Another indicator of when it is indicated to play ...h6, ...g5 is when subsequently you can play ...Nh5 or ...Nf5 and the B on g3 cannot escape to another square. When on the following move you play ...Nxg3 you have accomplished at least 2 things. One you have gaine the minor exchange (trading your N for a enemy B). The second thing you have accomplished is to gain the advantage of being the player with the Bishop Pair (a very strong attacking duo in the hands of a knowledgeable player.)
Another thing you will have accoplished if your opponent's king is castled on the side where you executed the minor exchange, is you will have doubled the pawns in front of his King's castled position. A weakness that can be exploited.
When can black legitamitely play h6,g5 to thwart off a pinning bishop? I know there is theory when this is okay to play for black but I just forget it.
Thanks,
Dave