Quote: 5.6 If a player engages cruise engine or docks during a PvP fight, this player is considered fleeing. The fleeing player must leave and may not re-enter the system where the fight took place with any of the characters on his/her account(s) while the enemy (player or players involved in the fight) remains in the system, but no more than 4 hours.
That right there covers it. The first player to hit cruise is considered fleeing. It doesn't matter if you're trying to escape, or catch up to someone, if you're the first one to engage your cruise engines, you're out of the fight. If your target hits cruise to get away from you, the you can cruise to chase them, but you can't cruise to catch up to someone fleeing on thrusters only.