The expressive hypothesis is kindred to the repressive hypothesis which was, as some may already know, posited by Foucault. The repressive hypothesis says that whenever something, e.g., sexuality during the Victorian age, is perceived as "repressed," it is actually being talked about all the more plentifully. So, although the common lore is that sexuality was repressed during the Victorian era, it was actually the subject of endless discourses on containing, managing and purifying the sexual body, particularly of females, homosexuals, ethnic populations and adolescents.
Now, the expressive hypothesis is somewhat similar to the repressive hypothesis, to the extent that it suggests that the more you express, the healthier -- emotionally, psychologically, spiritually -- you will be. It is like a talking cure or talk therapy, in a way. Expressivism can be a therapeutic exercise without, in any way, diluting its intellectual or theoretical force. You express so what is within the heart and mind can come out and engage the world communicatively.