Complex sentence: This sound change affects the first element of the diphthong, and raises it from open, similar to the vowel of "bra", to near-open, similar to the vowel of "but".
Split into simpler sentences: This sound change affects the first element of the diphthong. Through raising, the beginning of the diphthong changes from open, similar to the vowel of "bra", to near-open, similar to the vowel of "but".