Stack attack?

Quick question: There is a game option for stack attack, which was turned off by default. In Civ4 I dont recall any option for this, it was always stack attack, so I would assume it works the same.

Does anyone know for sure what the combat mehcanics are with it on and off?

No combat mechanics change, it's just simplicity(So you can have 10-20 units attack with one click instead of clicking for every single unit). It exists in Civ IV also.