Got this from another forum which I think explains what you are trying to say best:
However, this is NOT a feature in many cameras. Perhaps only Nikon cameras, I do not know. Definitely not TS camera, which is a Canon 550D. Neither is it a feature on my camera, or many cameras out there. Personally, pressing the EV comp button in M on my camera alternates between letting you select shutter speed and aperture.Regarding specifically Nikon D700
It does not change your exposure, this is left to your manual settings. However it shifts your metering.
So if you set your +/- to -1 the meter will guide your shot to 1 stop underexposed.
Also, my sentence is phrased perfectly fine. Please read again. The fact that you said EV comp works ALSO in P mode, implies that it works in both M and P, which is not the case, keyword being "also".
I hope this makes sense.