Firstly, Illič-Svityč's law refers to the rule according to which Proto-Slavic thematic neuters accented on the first syllable become masculines. Compare:
This rule is important because it operated after the influx of Proto-Germanic/Gothic thematic neuters, which all became masculines in Proto-Slavic. Late Proto-Germanic (after the operation of Verner's law) had fixed accent on the first syllable. Compare:
Secondly, Illič-Svityč's law refers to the rule according to which all masculine o-stems in Proto-Slavic generalized accentual mobility (accent paradigm 'c', as opposed to expected accent paradigm 'b').
Older literature suggests that this was not a Common Slavic innovation, and that there are exceptions in some Croatian Čakavian dialects of Susak and Istria, which have retained the original accentuation. This has been recently disputed.