If you're talking about the Japanese ATB** script in his demo, it edits the Scene_Battle script and adds an Active Timer BBattle bar at the bottom of each battler's stats. The bar's progress shows when your battler is ready to attack, and this script will pause the action until either an enemy battler attacks or one of the ATB bars becomes filled. Removing it removes that yellow ATB bar at the bottom and makes the action just as instantaneous as the default battle system.
His version 2.5 system hardly touched the default system, mostly a visual edit than anything else. Both of 'em merely SIMULATE the ATB experience, as you still take turns attacking. From what I've heard, version 3.0beta is still a work in progress and is rumored to have a couple of bugs in it. Version 2.5, on the other hand, is referred as the more stable and reliable version.
Oh, and many scripts that tweak out the default battle system work with it.