Off is said to be better for passing or intown driving becuase it keeps the rpms higher in each gear and as AZ ZBum stated it doesn't let it go past 3rd gear. When on the freeway cruising you want the rpms lower and the 4th gear to help with gas mileage. What you'll find is that if you are going 65mph on the freeway and want just nail it for what ever reason if you floor it the kick down switch will drop it into the next gear and as long as you are flooring it will keep it in 3rd until it reaches 6000rpm (5200rpm if you have an 84). Then it will shift to 4th. However you can just kit the OD and the tranny will shift it down into thrid for you without you having to floor it and then you can regulate how much to press the gas pedal to pass someone. Honestly there really isn't much of a difference becuase the kick down switch will always get you moving faster. I guess you could say that if you wanted to always be ready for a race to keep the OD off that way rpms would be higher. It would be the equivalent to always staying at 3k or higher before shifting in a manual trans just so that you would be in your power band. I always keep mine on and only switch to off when I'm passing someone uphill on the freeway. Like AZ said, if you leave it off becuase it won't upshift past 3rd you will not be able to go much faster than 100mph. So while racing when you get to the end of 3rd turn it on so that the tranny will go into 4th.