How did the High German consonant shift divide Low German and High German dialects?

The High German consonant shift, which occurred around the 6th century, fundamentally divided the West Germanic languages into two distinct dialectal groups based on sound changes.12

In the southern highlands, the shift transformed the voiceless stops p, t, and k into affricates (at the beginning of words or when doubled) or long fricatives (after vowels). In contrast, these sounds remained largely intact in the northern lowlands, maintaining a pronunciation closer to that found in modern English.23

This linguistic boundary is famously illustrated by the Benrath line, an isogloss that marks where these shifts occurred. A primary indicator used to define this division is the maken/machen line:3
* Low German: Retained the original k sound (maken, hopen, bīten).2
* High German: Shifted these to fricatives (machen, hoffen, beissen).2

Because these shifts spread north from the south to varying degrees, they created a complex bundle of isoglosses known as the "Rhenish fan," which serves as the primary distinction between the High German and Low German speech areas.23