glad you asked - (I'm one of co-founders of EyeQuant.)
In a nutshell, EyeQuant's predictions use a model of human attention that is based on hundreds of empirical eye-tracking studies, which were conducted at the Neurobiopsychology Lab at the University of Osnabrueck, Germany - also, we're cooperating with Caltech and USC.
The models only represent the cultural background of tracked subjects - it's strictly data-driven. Our current model is representative for western subjects; it can't predict, e. g., people who usually read Farsi. It also only predicts the first 3-5 seconds.
Here's the method: In the eye-tracking studies, we present subjects with websites from different categories and track their fixations - more importantly, we also analyze the statistical features of the fixated spots (around 50 of them to date): color contrasts, luminance, shapes and location, to name a few of the very basic ones.
In the next step we build models based on the weight of these features and evaluate them with other eye-tracking studies as a gold standard. One eye-tracking study typically predicts another one with 90-95% accuracy for the first few seconds (ROC). With EyeQuant, and after 5 years of foundational research, we achieve around 84% predictive accuracy - that's pretty close to what you'll get with an empirical study, but the results are available within seconds.
Let me know if you have more questions!