The Mixer Test is something many people are scared of and wary about. It is sometimes made to sound technical and like you may need a PhD to figure it out. Quite simply though, it is a very clever invention which will allow you to maintain your kites flying characteristics throughout its lifespan. As lines stretch (which they always do) you will need to compensate for this.
Here is a video of Flysurfer lead designer Armin Harich explaining the Flysurfer bridle and how to perform the mixer test.
Here is a video of Flysurfer team rider Eric Fernholm explaining the mixer test and how to troubleshoot certain flying characteristics.