It's strobing the backlight.
All strobed backlights in all techs will make the screen "darker" when enabling strobe mode.
This goes for Lightboost, Blur reduction and ULMB and Turbo240.
What you need to do is adjust the persistence lower or higher to compensate for it. Lower persistence (lower millisecond strobing) gives more intensity of motion blur reduction (in the 2430T, they actually reversed the values and call it "intensity" instead of "strobe duty" to have it make more sense....an "intensity" of 30 means MOST blur reduction=darkest screen, etc. More blur reduction means the backlight is strobing on and off faster which means less brightness.
This article may explain more. There's also a link to how lightboost works on it, too.
http://display-corner.epfl.ch/index.php/BenQ_XL2411Z
You can use the Vertical Total 1500 tweak to improve brightness (although this raises the persistence which slightly reduces the effect of blur reduction at the same strobe duty value, BUT has a HUGE effect in reducing (actually, moving the bottom side) of the strobe crosstalk farther down or OFF the screen (strobe crosstalk is an artifact of pixel transitions during strobe mode, ala "strobe pulse"; raising the Vertical Total to 1500 will, assuming a strobe duty of 006 (1 millisecond if using a VT tweak), will drive the bottom side crosstalk almost completely off the screen (and at refresh rates of 85 hz or lower, will have the crosstalk COMPLETELY off the screen), while not using a VT tweak will have the bottom third of the screen a rather double image blurry mess (this usually isn't a problem if you're playing FPS games with a hud covering the bottom).
There are other step by step posts that explain how to use the Vt tweaks. It's in the how-to, and Chief really should make a sticky for it
The reason CRT's don't get 'darker' is because they are designed by -nature- to strobe. It's built into the technology. LCD monitors were never 'designed' to strobe...it's more of a 'hack job' with the electronics (not that hard to do, but controlling crosstalk with how overdrive works IS difficult to do correctly) with current panel technology.