I think its well explained by Stuart Cummings (a good addition to Sky panel). If an attacking player is in front of the play the ball when its played he cannot advance downfield and get involved unless he retires past the spot of the ptb. A regular offside from the kick is where he is behind the ptb when the ball is played but in front of the kicker
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Downtown is given in this clip as the downtown offside penalty mark is where the previous ptn is. A regular offside is 10m from where the catcher catches the ball or where ball is picked up