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 suggestes that this was not Common Slavic innovation, and that there are exceptions in some Croatian Čakavian dialects of Susak and Istria, which have retained the original accentuation, but these have been disputed recently.