The guidelines for Assistant Referees is offside is priority number one and there is a good reason for that, because he’s on line with the second-last defender.
But the referee can support him. How? A few examples:
Situations where a player gives a long ball and someone heads it forward. Who touches it? Nobody does?
It’s quite difficult
The referee’s judgment should take into account the following criteria.
Did the defender unfairly hinder or impede the movement of the attacker by his hand/arm contact?
Did this contact negatively influence the advance of the attacker or the progress of the team attack?
Was this contact designed to hinder (or was it accidental)?
Did the attacker initiate the illegal