The grey side is embossed to increase friction. Extra roughness helps in the beginning to withstand sliding off the head, which allows extra head movement. Once on your head, there is not other way that sitting straight. Complete your therapy while doing daily stuff like working on computer, watching TV or walking. Only five to ten minutes a day, any time, at your own pace.
Small and discreet size can be placed anywhere as gentle reminder to pay attention to your posture.