HiTech.. here's my findings.
looking closer i am assuming it is a design feature
all planes tested (190 d, a, f; 109 g, e, f) when you set the RPM lower than the full MAN is (example 190d RPM to 1900 or 2000) it makes the MAN go down a bit without any input on the throttle itself
i assume egines would sort of do that, but not sure
anyway, if you set the d9 for example to 2000 RPM the MAN is a bit below the max. hitting the WEP button then has no effect. only when you keep the RPM's high enough for the MAN to not self adjust will WEP bring the plane to full emergency power.
does that make sence? i hate trying to type out something i can vocalize in 30 seconds LOL