I believe McWhorter had a bunch of verbs that didn't have cognates in non-Germanic IE languages. They were all strong and matched Semitic roots.

So, that means that Proto-Germanic came into contact with a Semitic language. Which one? And how did all those other IE languages get ablaut which most historical linguists trace back to PIE?

Basically, umlaut is the changing of a vowel because in a subsequent syllable there is an i. Ablaut had to do with the variation in vowels (such as that in Germanic strong verbs) caused by some other phonological process, e.g., the shifting of tone or stress (depending on when it happened).

Ceci n'est pas un seing.