This is the one Serpent Breathing form named for anatomy rather than motion. Every other numbered technique in the style describes what the blade does (斬り, "slash") or how it moves (蜿蜿, "winding"); 狭頭の毒牙 instead names a body part — specifically, the tapered skull shape that herpetologists use to visually distinguish venomous vipers from harmless snakes at a glance. Attaching that identification detail to an attack name implies the strike itself is meant to read as a bite from a specific, dangerous kind of snake, not a generic slash reframed as "serpentine" — a more anatomically literate naming choice than the style's other entries, and one that fits Obanai, a former viper-charmer's apprentice raised in a shrine that worshipped snakes, for whom this vocabulary would be second nature rather than poetic flourish.
毒牙 (dokuga) is common enough as a word that most Japanese readers will recognize it outside any snake context — it's the standard word for a venomous fang in ordinary speech and appears figuratively all the time (a corrupt official's 毒牙 reaching toward a victim, for instance). Pairing an everyday word (毒牙) with a specialist one (狭頭) mirrors a pattern seen across both breathing styles in this dictionary: Mist Breathing's forms mix classical-poetry rarities (垂天, 朧) with more transparent compounds (八重霞); here, Serpent Breathing does something structurally similar by mixing a snake-anatomy term few readers would know unprompted with a poison-fang word everyone already does.
VIZ's "VENOM FANGS OF THE NARROW HEAD!" is a rare case of a fully literal, un-simplified translation across this whole ten-entry set — every element of the Japanese (narrow, head, venom, fangs) survives intact in the English, likely because the phrase reads naturally as an ominous attack name in English without requiring any cultural gloss, unlike Mist Breathing's classical allusions. The corpus's automated bilingual alignment has no English text at all recorded for this occurrence (a known gap in this stretch of Vol. 19); the quote above is confirmed by direct visual check against the real VIZ tankōbon page, one page after the Japanese.