For example, hunchback in moderate concentrations is an activator of kruppel, but a suppressor of kruppel in large concentrations.
From what I've seen in literature, that's because the kruppel's enhancer has several binding sites for hunchback: some responsible for activation, while others for suppression. But why would hunchback in moderate concentrations bind only to activation sites, and why would higher concentration "change its preferences"? What chemical mechanism is responsible for this preference?
P.S. I provide hunchback and kruppel as an example, but this is a general question: how could the same transcription factor be both an activator and suppressor of the same gene?