OP, could you replace that “Slave” with “Slavic” in the title? It represents better the content of the video, including its title.

I’m watching it now, by the way. It’s about the impact of /VjV/→/V:/ contraction in the Western dialects of Common Slavic, and it claims that it’s a factor behind the fragmentation of Common Slavic. I’ll comment further as I finish watching this video.

The video is a bit messy but it has a lot of good content. I’ll kind of summarise and explain it here.

Around 1000 CE, Proto-Slavic already fragmented into multiple varieties keeping some mutual intelligibility, so it was still a single language. Something like “yeah, I understand you, but you speak really funny.” That period is known as Late Common Slavic.

The video claims that one set of sound changes, in special, was the last nail in the coffin for that unity: it was the VjV→V́ contractions (where “V” = any vowel). This happened in six phases:

1. Phonetic phase: if both vowels in VjV are front high (ь ъ i y), simplify it to V́.
2. Phonologic phase: if both vowels in VjV are identical to each other, simplify it to V́.
3. Morphonologic phase: if the vowels in VjV are similar (for example: a back open and a back mid vowel), simplify it to V́.
4. Morphologic phase: if the vowels in VjV are slightly similar (for example: a back open and a mid front vowel), simplify it to V́.
5. Morphologised phase: even if the vowels are extremely distinct, most leftover VjV sequences get simplified to V́.
6. Yer phase: if there’s still some ь or ъ leftover, ditch it.

The last phase might sound “weird” in contrast with the first five, but think on it this way: Late Common Slavic had two vowel lengths, full and short (the yers). Those changes introduced a third vowel length (long vowels); as a result, the shortest vowels of the bunch got the shortest end of the stick, and got elided into nothing.

Those changes had a lot of impact on declensions, conjugations, and even the accent system, but there’s a catch: while all Slavic languages underwent the first phase, the other phases are more restricted, and might not apply to a specific Slavic language. And the further (geographically speaking) that you go from Czech, the less likely is the language to adopt a certain phase. So for example, you see most of those changes in Polish and Silesian, but barely any of them in Russian, and only some of them in the South Slavic languages.

[Relevant detail, not mentioned in the video, about the South Slavic languages: they used to be spoken further north, rather close to where Czech is spoken.]

So this six sets of sound changes are centralised in a place, adopted only partially by the Late Common Slavic speakers, and have big repercussions on the rest of the system; that’s a recipe for strong divergence. In this aspect it’s a lot like Italian undergoing Vs→Vj, or Old English merging some word ending vowels - simple sound changes triggering deeper grammatical changes.