It's actually too complicated to explain.
But the easy version is, the monitor reverts to using a 60 hz backlight strobe width for the backlight (NOT the refresh rate--the strobe width (persistence) whenever it is being run out of specifications.
There's a comment on that here.
http://display-corner.epfl.ch/index.php/BenQ_XL2411Z
I'm going to use 100hz as a fast example.
Without a vertical total tweak at 100 hz, strobe duty 001 would be 0.1 ms persistence. and the max strobe duty 030 would be 3.0ms persistence. (0.1ms persistence added per point). The higher the persistence, the brighter the screen and the less blur reduction effect you have. The lower the persistence (strobe duty value or that slider) the dimmer the screen and the greater effect of blur reduction.
Persistence is basically how often the backlight is turned off and on (this is what strobing /blur reduction does).
You can see the display corner website says that the minimum backlight pulse width at 100 hz is 0.1ms
So a strobe duty of 010 would give 1.0ms persistence.
Now VT tweaks force the monitor to use the 60 hz backlight pulse widths instead of 100 hz. (Do NOT confuse this with refresh rate!).
Normally at 60 hz refresh rate (if you are using SINGLE STROBE "ON" through the service menu or have the box checked in the windows utility; the windows utility just turns on single strobe for you), the lowest persistence is 0.167ms and the highest persistence is 5.0ms.
(30 levels of strobe duty. 0.167 x 30=5).
Yes, the backlight is on/off for a 'longer' length of time at 60hz refresh rate (0.167 ms for strobe duty 001) than it is at 100 hz ( 0.1ms ).
Using a VT Tweak, for some reason, forces the monitor to switch to the pulse widths for 60 hz. Thus, whether you are at 100 hz refresh rate or 120hz refresh rate, strobe duty 006 now becomes 1.0 ms persistence instead of the values shown on display-corner. Thus, the screen becomes brighter. (without a VT tweak, strobe duty 006 would be 0.48 ms at 120 hz, but WITH a VT Tweak it's 1.0ms--the screen is TWICE as bright
Well, WHY?
Uh...tbh its something to do with the monitor's own scaler thinking it's running at some new resolution, because the vertical total has been changed (you can only see this with version 4 firmware, in the "second" service menu). This scaler change defaults back to 60 hz, and it also messes up the "display mode" aspect settings, causing aspect to suddenly be available and used at 1920x1080 when it shouldn't be available (the scaler thinks the horizontal width is 1280 with a VT 1500 tweak or 1600 with a VT 1350 tweak. whatever.....)
The easiest way to see if a VT tweak has actually worked or not is to check the OSD Information. if it says 60 hz, it worked. If it says 75, 100, 120 or 144 hz, it didn't.
