The notes mention the consonant change here. Before -ach (-er) and -a(f) (-est):
b becomes p e.g. gwlyb to gwlypach (wetter)
d becomes t e.g. drud to drutach (more expensive)
g becomes c e.g. teg to tecach (fairer)
I don't know why the notes say its "softens" the sound, as it actually "hardens" the sound. That's why in Welsh we call it caledu (hardening). It's kind of like the opposite of the treiglad meddal (soft mutation).