I've been playing this since it came out now, and have seen players who lean left when they strafe left, and lean right when they strafe right. They manage this while shooting me in the head every time as well

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I'm not that bad at the game, I do rather well in most servers, but in one particular server there's always 2 guys who beat up on everyone else. I'm not a bad loser, but they refuse to say whether they're "leanbinding" or not. I find it hard to believe that they can press 2 keys together, then another 2, then the first 2 again repeatedly, all while being very accurate at the same time.
My theory is they've bound the strafe left and lean left commands to the same key, and another for moving to the right. How do you do this, and is it looked apon as being unfair, or even "cheating"?
Cheers.