It's from Dragonstar, and since it's OGL I'll reprint here. Go buy the book. It's really quite cool
Pressing Attack [General]
You are skilled at seizing every advantage and keeping your opponent on the defensive.
Prerequisite: Combat Reflexes
Benefit: This feat allows you to follow an opponent who tries to step back from an area you threaten. The opponent must be in an area you threaten at the begining of his action. If the opponent takes a 5-foot step to an area you do not threaten, you may immediately take a 5-foot step of your own to any unoccupied space where you again threaten the opponent. If no such space is available, you cannot use this feat. You may only use this feat once per round.