Foxes try to attack mobs that hurt the player they trust. A lesser known fact about this game mechanic is that foxes only check if the target mob is nearby, and will attempt to attack even when the actual player getting damaged is thousands of blocks away.
The transmitter damages the player with a TNT ignited using an arrow from the skeleton. This will make the fox run towards the skeleton and stand on the pressure plate. The fox needs to be on the pressure plate for a certain amount of time before the receiver detects it, something it doesn't do usually because of the wolf scaring it away.
You need witches and iron golems because foxes only retaliate against mobs that are attacking something.
I'm pretty sure this is a new concept, but please correct me if I'm wrong.
by polaris3506