The target is V2+15. If you rotate slowly then you'll naturally blow past that. As you do the flight directors will command that higher speed, but only up to a max of V2+25. (My understanding is that) the rate of rotation is irrelevant, the speed you get to during rotation is what determines whether you end up climbing out at V2+15, V2+25 or somewhere in between.
So it doesn't "register a rotate", other than weight off wheels registering a takeoff - it registers airspeed, and you can find that info on the Airspeed Indicator. 😜
Last edited by rudestuff; 8th Dec 2021 at 19:00.