Complex sentence: Classical right of way requires a fully extended weapon arm that continuously threatens the defender's valid target area in order for an action to be considered a valid attack--any withdrawal of the weapon arm during the attack jeopardizes the fencer's claim to right of way.
Split into simpler sentences: Classical right of way requires a fully extended weapon arm that continuously threatens the defender's valid target area in order for an action to be considered a valid attack. Any withdrawal of the weapon arm during the attack jeopardizes the fencer's claim to right of way.