How did Verner’s law explain sound differences in early Germanic languages?

Verner’s law explains apparent exceptions to Grimm’s law by demonstrating that the position of the accent (stress) in the original Indo-European language influenced sound changes in early Germanic languages.1

While Grimm’s law established that voiceless stops (p, t, k) shifted to voiceless fricatives (f, th, h), Karl Verner observed that these sounds became voiced (b, d, g) under specific conditions. Specifically, if the original Indo-European accent did not fall on the root syllable, the resulting non-initial voiceless fricatives became voiced when positioned between voiced sounds.1

For example, the Sanskrit word bhrātar (accented on the root) corresponds to the Gothic brōþar, maintaining the voiceless fricative. Conversely, the Sanskrit pitā (accented on the final syllable) corresponds to the Gothic fadar, where the t shifted to a voiced d due to the placement of the accent. This discovery was pivotal for the Neogrammarian school, as it provided evidence that phonetic laws operate with regularity.12