Speaking about harmonic domains, usually we mean a sequence of syllables in which harmonic vowels must belong to the same harmonic class. However, in the languages discussed in §4.1, the actual domain of harmony can be shorter than the potential domain, which is understood as a sequence of syllables in which harmonic vowels may belong to the same harmonic class. In these languages, one of the harmonic classes is preferred (defalult). the members of this class can follow the members of the other class as well (but not vice verse). That is, there is a kind of vacillation, every time when a harmonic feature can spread from one syllable to another, there is a chance that it will not. Moreover, the more distant is the syllable from the initial one, the lesser is the chance that a member of the dispreferred class will follow the previous syllable containing a member of the dispreferred class (and after some number of syllables, the vowels belonging to the dispreferred class are completely prohibited). Domains with harmony fading syllable by syllable we will call harmonicity slopes. 

The phenomenon is basically described by Wiik (1988: 189–195) where he uses the metaphor of the electric torch with a discharging battery. According to this, initial-syllable vowel is the torch, the light of this does not reach so far as earlier (shorter domain), the light is weaker in some spots (centralized allophones instead of back and front ones) and the torch works occasionally (vacillation).