According to the law, Slavic words have a circumflex on the root vowel (i.e., the first syllable of a word) with a Balto-Slavic acute if that word had a mobile accent paradigm in Proto-Slavic and Proto-Balto-Slavic. Compare:
Meillet's law should most probably be interpreted as polarization of accentual mobility in Slavic, due to which accent in the words with mobile accentuation had to be on the first mora, instead on the first syllable (in places in paradigm with initial accent). This is the reason in the words belonging to mobile paradigms in Slavic accent shifts from the first syllable to the proclitic, e.g. Russian accusative singular of mobile-paradigm gólovu, but ná golovu 'on the head', Serbo-Croatian glȃvu, but nȁ glāvu.