Transmission and reflection of light incident normally onto a layer of medium having a helical structure with the axis perpendicular to the interfaces are under consideration. Mechanisms of anomalous absorption of radiation in periodical helical media are discussed. Existence of two mechanisms of absorption suppression and two mechanisms of anomalously strong absorption, respectively, which essentially differ from each other, are shown. Influence of group velocity onto anomalies of absorption has been studied. These two mechanisms act both on frequencies of diffraction of light on a structure of medium and on frequencies of diffraction of light in limited volume. It is shown that these mechanisms of anomalous absorption are of fairly general character. They can also manifest themselves in interaction of radiation with various periodic media. The specific features of dynamics of these mechanisms are studied. The influence of layer boundaries onto a group velocity and wave vector of total wave in the specimen have been investigated. Strong and weak resonance absorption of light on the spectral line of integral absorption in isotropic dielectric slab has been discovered. In this case the specific features of modulation of total wave in the specimen have been investigated, too.