Umlaut, Vowel Harmony etc

Can anyone tell me how changes towards such a system work in practice? I
understand that vowels influence neighbouring vowels, but for instance
is it usually progressive or regressive, and how does it stop? I mean,
if for instance one vowel influences a neighbouring vowel to becoming
rounded, and then that vowel does the same, then roundedness would
spread throughout the language before too long, so the application of
such rules must be limited somehow since we don't find any languages
with all rounded vowels, or all high vowels, etc that have taken harmony
right to the extreme.

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