According to the law, Slavic words have circumflex on the root vowel (i.e. the first syllable of a word) with Balto-Slavic acute, if that word had mobile accent in paradigm in Proto-Slavic and Proto-Balto-Slavic. Compare:

• acute on Lithuanian gálvą, A sg of mobile-paradigm galvà 'head', vs. circumflex in Slavic (Croatian glȃvu, Slovenian glavô, Russian gólovu)
• acute on Lithuanian sūnùs, A sg sū́nų, vs. circumflex in Slavic (Croatian sȋn, Slovenian sîn)

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 why in the words belonging to mobile paradigms in Slavic accent shifts from the first syllable to the proclitic, e.g. Russian A sg gólovu, but ná golovu 'on the head', Croatian glȃvu, but nȁ glāvu.

References

• Ranko Matasović (2008). Poredbenopovijesna gramatika hrvatskoga jezika. Zagreb: Matica hrvatska. ISBN 978-953-150-840-7.