Heres what the source code has to say:
dummyconsvar is a Hack to clear all changed flags after game start.
* A lot of code (written by dummies, obviously) uses COM_BufAddText() to run
* commands and change consvars, especially on game start. This is problematic
* because CV_ClearChangedFlags() needs to get called on game start \b after
* all those commands are run.
*
* Here's how it's done: the last thing in COM_BufAddText() is "dummyconsvar
* 1", so we end up here, where dummyconsvar is reset to 0 and all the changed
* flags are set to 0.
*
* \todo Fix the aforementioned code and make this hack unnecessary.
* \sa cv_dummyconsvar
* \author Graue <graue>