Consider having a sideline player whose role is to speak to the player closing in for the force about where the handlers are. The positiong (not the pace) of this player in shutting down the centring throw to another handler is key.
This movement can seem counterintuitive. Encourage players to realise that once the sidetrap is broken and the disc is back in the centre, their priority is returning to the lanes to prevent easy passes through the centre.
Keep returning to the purpose of this set, which is to avoid giving up easy yards, either through the centre, or down the sidelines. This will make fanning out to prevent offence progression more understandable
Encourage players to communicate loudly when they feel a line trap should happen. Emphasise that there is no harm in commiting to a line trap pre-emptively as long as the players taking out the up-the-line throw commit aggressively.