Set f4 to AE-L Hold
Obtain the exposure you want an press AE-L. The metered EV will be stored for some time (you can set the timeout via c2)
Easy Exposure compensation still works and you can vary the aperture in A-mode (or the speed in S-mode) and the D90 will compute the speed (or aperture) for you.
This helps in several situations where you’d consider shooting manual, but retains the convenience of adjusting aperture/speed by a single dial flip
I took me some minutes to learn it - embarrassing that I ignored this option for more than a year.
*Actually they cheated, the Pony II had a fixed shutter speed…But EV is easier to explain than Sunny 16.