adverb
facing slightly downward; with one's head somewhat lowered
Describes a posture where the head is tilted slightly forward, often used with ～で or ～に. Common in descriptions of body language.
彼は俯き加減に歩いていた。
He was walking with his head slightly lowered.
俯き加減で話すのは自信がなさそうに
Speaking with your head slightly down makes you look unconfident.