At some point in the early medieval period the reduced vowels stopped being pronounced, but not all of them, and which ones fell was determined by position rather than by anything about the individual word.
The rule, worked out in the nineteenth century and named after the scholar who formulated it, counts jers backwards from the end of a word. A jer in a syllable followed by another jer is strong and survives, changing into a full vowel. A jer not so followed is weak and falls silent. Since the counting alternates, the result in a word with several jers is that they fall and survive in alternate syllables.
The consequences reshaped the languages. Words could now end in consonants, which they never had before. Consonant clusters appeared that had previously always been separated by a vowel. And a very large number of words acquired a vowel that appears in some grammatical forms and vanishes in others, depending on what followed, which is why Slavic grammars all have a section on fugitive vowels.
The orthography lagged behind by centuries. Scribes went on writing jers where they were no longer spoken, because that was how the words were spelled, and Russian was still writing a hard sign at the end of every consonant final word until 1918. Removing it saved a noticeable proportion of the paper in a printed book, which was one of the arguments actually made for the reform.
The jers therefore illustrate the whole relationship between sound and spelling in this tradition: the sound goes, the letter stays, and the letter finds a new job.