Per Keith Hackett, writing in the Observer a couple of years back:
The law really is simple and well defined. First and foremost, it is important to know the key principle: it is not an offence in itself to be in an offside position. Assistants will not flag the moment someone strays offside. A player is only penalised if he then becomes active.
The source of most confusion is clearly in the definition of 'active'. Pundits keep falling back on the dictionary definition of the word, or their own version of it, rather than the one set out in the laws.
To be clear, the definition, in the laws, is this: in deciding whether to flag, assistants must watch out for three things, any one of which would make an offside player active.
First, is the offside player interfering with play? As advised by the IFAB since 2005, that means playing or touching the ball. Attempting to play the ball does not count - he must actually play or touch it.
Second, is the player interfering with an opponent's ability to play the ball, by clearly obstructing the opponent's line of vision or movements, or by making a gesture or movement which, in the opinion of the referee, deceives or distracts an opponent?
And third, is the player 'gaining an advantage'? This last point is specific, and is not what Match of the Day seem to think it is. It applies only to an offside player playing a ball that rebounds to him from an opponent, the post or the crossbar. If he does not play the ball from the rebound, then he is not penalised for being in that offside position. Nothing else counts as 'gaining'.
And that's it. If a player ticks any one of those three boxes, he is offside. The three-part definition is remembered as 'PIG' - if a player doesn't Play, Interfere or Gain, he is fine.
So the nub of the issue is did the ball 'rebound' from Koscielny. He clearly wasn't in control of the ball and, if it had gone through to 'Chesney', he could have picked it up as it couldn't have been construed as a back pass.
So what is a rebound? Despite Hackett's assertion that the law is 'well-defined' this is clearly not the case here and is open to interpretation.
And talking of 'PIG' - a pig could probably have refereed the match far better than Lee Mason.
As a final point, Villa's first goal in the match around Christmas should have been disallowed as Carew was in an offside position and interfered with Fabianski's ability to play the ball by blocking his line of vision. Do the refs really understand the rules?