The button lets you shift between 5th and 6th and lets you keep the rpms where you need them if you are running at say 70mph. I do run a stock hpop with these injectors and have a 6in lift with 37" tires and can still haul ass with this setup. It all comes down to getting the tuning right...
Bump the shift points up and take the overdrive off and have the tow haul button option included in the tuning just like Swinky said.. Made a night and day difference in mine. I run a s366 with 225/100. Spools like a stock turbo. I run the SPD tunes.