The difference was between an attention getting flag and an offside flag... I took an exaggerated step onto the pitch (as per instructions). It was only my second or third line since I'd qualified. The main thing was, I'd gained the attention of the referee, who came over to me. We spoke about the scenario and both agreed on the outcome.
Surely an "attention getting flag" is not for offside (or ball in/out of play, or goal/no goal).
If it is a potential offside goal and you are not sure, you stand.
The signal for goal is either: run towards half way, or flag up to signal the ball has crossed the line if not clear to all, and then run.
If you wave an attention getting flag, you are signalling there has been a different offence and you will cause a great deal of confusion (of course I have done this
)
It is up to the ref to look for the assistant - if the AR is standing, the ref knows to confer or to answer the "question" from the AR with a clear signal if the ref can decide or has decided goal/no goal. If the AR is waving, then, depending on which hand the AR has the flag in, he/she/you might cause a penalty to be awarded.
(I have not been trained in the UK but I understand this is universal, no?)
Edit: in the clip, the AR is not "wrong" to flag offside, but the smart move would have been to stand. The players may have still reacted the same way of course... I think the assessor would knock one off for the signal there...