A player will be called offside if
a) she is in an offside position (closer to the goal than the second to last defender) WHEN
b) her teammate kicks the ball forward, AND SHE THEN
c) gets involved in the play (by receiving the ball or moving toward the ball or otherwise involving herself in play in the judgement of the refs).
Note: What’s important is where she is when the ball is kicked. Not where she is when she receives the ball, or where she is when the rest of the players notice her. This call is the responsibility of the Assistant Referee (also knows as Linesperson), who stays with the offside line. The offside line is defined as the second to last defender or the ball, whichever is closer to the defensive goal.
AR’s are instructed to wait to make the call until the player actually gets involved in the play, so they don’t call someone "off" who doesn’t get involved. This can result in what seem like late offside calls. We know you don’t like running for nothing, but we also know you would not like us to make a wrong offside call.
By the way, there is only one "s" in OFFSIDE.