this feature is two of four count-down modes. you need to be in the count-down mode, p+t (i think, but it might be s+t) cycles thru the modes. then you need to press p+s to cycle thru the count-down modes. there is preset 1 and 2, and auto lo and hi.
preset 1 is a preset time that is smaller than preset 2. these presets stay the same and always count down that set amount (say 30sec vs 1min).
the auto settings also have preset times but they are a base to which the greddy timere adds secs depending on revs and driving time. auto lo starts adding time on to the lo preset at 3k rpm and auto hi does the same but starts adding at 2k rpm.
these modes are a great idea but are excessive when you do long drives. when i drive from brisbane to toowoomba the auto lo gives you about 10 min, and the auto hi gives you so many minutes the display won't show the exact no. just says 10+ or 20+ or something...
the rpm thing your talking about lets you use the timer as a rev counter. both these features are redundant if you're rev wire is not connected.