Text-based roleplaying (which is pretty much all online roleplaying, even in graphical virtual worlds, because the emote structures aren’t robust enough to handle interactions beyond social basics and PvP, much) takes a particular kind of grace and courtesy. You take your turn, writing what your character does — make your “pose,” it’s called — and then wait for others to respond. The grace comes in making sure your pose invites a response from the other player (and his/her character) but does *not* impose action on the other character in any way.
That is, you can say, “Negaduck whips out a shotgun and tries to shoot Hannibal Lecter.” (Or even, perhaps, “takes a shot at” or “shoots at Hannibal Lecter).
You cannot say, “Negaduck shoots Hannibal Lecter right between the eyes. Hannibal staggers backwards and sinks to the ground.”
The first example invites a response from player-and-character, and gives the player a chance to choose what the character does and have fun with it. You put your cards down, and wait for s/he to play his hand.
The second example totally disregards the other player and assumes control of his/her character. You put your cards down, steal his/her cards, and play them however’s best for you.
That there’s what’s called power-posing, and it pretty much marks you as a) an overly-enthusiastic newbie or b) a griefer ass, who’s interested in playing other *players* and grandstanding, not in the cooperative dynamic of good roleplaying.
Internet etiquette is an odd thing — really, no action has been taken beyond the writing out of a fictional scene between fictional characters. But, in the context of the game and the social interactions it proceeds by, it’s funny how offensive power-posers seem. It’s like the kid on the playground who always says he’s won or is winning, even when everyone else knows he isn’t. He’s playing by a set of rules totally different from those of the game everyone else is trying to play, but loudly insists that he’s playing the same game.