This is what the original poster wrote:
Please note you ONLY threaten decapitation on an AoO. Otherwise you are just making a normal role. This was worded horribly.The Heads Will Roll (Ex): At ninth level the range that a Vorpal Warrior threatens decapitation when using a vorpal weapon increases by one.
It either means the range at which you threaten an opponent is increased, which is distance or it means that that the vorpal weapon gains a +1 when you threaten someone.
Terms have actual meaning in game.