How did Latin loanwords reveal when the High German consonant shift occurred?

Linguists determine the timing of the High German consonant shift by analyzing the phonological state of Latin loanwords borrowed into the West Germanic languages.

Because these loanwords were integrated into the language at different times, they serve as chronological markers:

• Pre-shift borrowings: Words borrowed before the shift occurred show the effects of the sound changes. For example, the Latin pondō became Pfund in German, and tegula became Ziegel. These forms demonstrate that the words were already present in the language when the shift transformed voiceless stops (p, t, k) into affricates or fricatives.1
• Post-shift borrowings: Words borrowed later, particularly during the Christianization of the Germanic tribes, do not reflect the consonant shift. Examples include predigen (from Latin praedicāre) and Tempel (from Latin templum), which retain the original sounds that would have been shifted had they been borrowed earlier.1

By comparing these two groups, scholars conclude that the shift occurred after the initial period of Latin contact but before the later wave of Christian-era borrowings.1