Respect your opponent's ability
Understand that your opponent probably had a reason for playing where he did, and don't blindly ignore moves that appear stupid at first. Additionally don't disrespect your opponent's ability by playing moves that rely on them severely screwing up to be effective, such moves are insulting and highly irritating.
Finally, always play honest moves against a weaker opponent, avoid trick plays. It is your duty to use your strength to show them the correct way of playing.
Yesterday I played a nine-stone teaching game against a beginner who won't remain at beginner level much longer. At a certain stage I said: "Look, I'm going to invade here (sansan), although it is a move that should not work. But if I continue to play properly, I lose this game (I was 30 points behind). Maybe my invasion will succeed, but it will be a nice exercise on how to punish an overplay."
And so we went on. With some hints, the whole corner developed into a ko, and with the compensation of the ko threat, and the yose, I closed the game down to a two-point loss. He was very pleased.
So, trick plays are very instructive teaching tools. If however, you use them only to crush weaker players, you're doing damage to your own game and to the game of your opponent.
I don't 100% agree. I find many beginners are way too respectful of the stronger player's moves. I think you need to be able to punish overplays if you want to get stronger, so I regularly play moves I know are bad when I play beginners.
The point is to set a problem for them and see if they can find the correct move. Of course there wouldn't be any point in this if you didn't analyze the game afterwards and explain the situation...
Coyotebd? I'm a good teacher then, as I often make horrible, terrible mistakes in handicap games. I then make noises and explain how I made such a terrible mistake. It gives the person I am playing against a chance to spot the mistake.
If they don't get it I point it out to them once they've played. Usually if I make the same mistake later they catch me before I've even seen it myself.
AnonLinguist: I prefer to leave holes in my defense for a while (say 10-20 moves) which I know are holes, but which I would like the weaker player to notice, too. So I present them with incomplete play, and hope they notice during the game on their own. It's worth mentioning afterwards what my source of seemingly limitless sente was, but during the game itself, I don't want to give hints other than playing the most curious gote moves.
In a real game, there are no hints, and if you can't spot it from looking at the board, you are in trouble. I have this notion that if there are things to reward a weaker player's effort in searching for a flaw, he will try harder to find them. A 10d can present flawless play to a 30k, but all that does is make the 30k worship the 10d, which is both useless and boring. It fosters a passive mindset, which does not engage at full force with the stones on the board.
Despite my attitude, I still come across people who just bow down and beg me to please show them how awesome I am (okay, maybe not that far...) and who afterwards tell me that they didn't try to play into a weakness I left for that purpose, because I probably had something awesome that I'd do in response. I just want to destroy that attitude, but it's hard to do when I really am much better at go than they are.
Then there's the times where I felt something was obviously not going to work at all, and my opponent asks me during review why I didn't try it, and then blinks as I show the obvious (to me) refutation.
I think my point is that you should respect your opponent, but in moderation.
SnotNose: I typically avoid trick plays and, more generally, most plays I can see are bad (with correct play by opponent). That is, I try to win handicap games (as White) based almost exclusively on mistakes by my opponent that I didn't induce. My reason for playing this way is that I'm thinking long term. I don't want to practice bad moves (practicing bad moves is worse than making them accidentally). Nevertheless, I lose a lot of handicap games as White. I don't mind, except during tournaments, at which times I wonder if I should practice a few trick plays just for tournaments...probably not. That would be inconsistent with my philosophy and for only vain reasons.
kvas: I have a friend (2d) who is playing the sort of Go that i would call hooligan style. When I play him (i'm 10k) he gives me 8 or 9 stones handicap and he can't resist the temptation for some overplay (well, he even uses it in a regular game). I find games against him rewarding, as I can develop my fighting skills very well, and if I manage to punish his overplay, it raises my confidence too. Wouldn't say that I like his style very much but for for developing ones tactical skills he's one of the best opponents.
Jade: As an addendum to this, do not assume your opponent will make the same mistake twice. It is very tempting, when an opponent ignores a good move, to play under the assumption that they will continue to miss it (and thus to ignore it in favor of smaller moves).
Alex: This is probably my biggest weakness. No matter how hard I try, I can't seem to break my own bad habit of losing respect for an opponent who plays ridiculous moves. It's particularly a problem when playing online, because of the crude "Internet style" found on most servers. My opponent will play abysmally during the fuseki and early middle game, giving me a large lead, at which point I start to think "man, this guy is weak" and stop paying attention. But of course, these players make up for weak strategy with stubbornness and strong tactics. If I'm paying attention, I can usually thwart their rabid late-middlegame attacks, but quite often I slip up because I wasn't giving him/her enough respect.
ilan: I have been thinking about this phenomenon recently. I have long thought that every game (go, chess, pool) has its particular rythm that one must come to terms with and that the above phenomenon is particular to "go rythm". In particular, in a typical game of Go, there can often come a very rapid transition in which strategical and intuitive decisions give way to purely tactical considerations. That is, you often see that groups which were connected or alive suddenly fighting for connectedness or life. From observation, this happens at every level, since I have seen a number of 7d games in which groups suddenly get disconnected or die suddenly near the end. Therefore, one must be prepared to quickly change hats from master strategist to master tactician. In some sense, this is unavoidable, because one can argue that the purpose of superior strategy is to give you a better chance of winning the tactics.
Anonymous: I'm curious how others feel about situations like this: Toward the end of the game Black is well ahead. White persists in making plays in black's territory which are bound to fail if Black answers correctly; in other words White's play is predicated on Black making a foolish error. White has nothing to lose so long as Black answers, but Black will lose a won game if he makes a mistake. I think this is poor etiquette on the part of White in that it shows a lack of respect for one's opponent's ability. Do others agree?
xof?: 9 stone games start in this position or close to it i have on more then one occasion given beginners 5 stones on a 9x9 board and killed everything they played i will normally go over the game showing how to defend the stones they have and how to attack the few stones i play but i don't think its wrong to fight for life i cant read out every position and sometimes make mistakes even now so i never care if someone wants to play it out to see if something works
if they can deffend it then it doesnt matter if they can't its your opportunity to teach them i would rather learn from a game then have my opponent let me win to be "polite"
Once I was playing a 9 stone handicap game with someone in a particular uni club.
After I placed my handi stones on the board and asked my opponent to begin,he started by tossing his first white stone onto the board.
My attention was elsewhere at that point so I wasn't sure if this had just happened. So I responded to this move seriously.
He continued to toss his next stone onto the board. When I realised what had happened, I was so shocked I couldn't move.
After about thirty seconds of staring at the stone, I closed the stone box, bowed and left the table.
I was unable to play a game with someone who shows no respect to his opponent, himself, or go.