一閃 is borrowed straight from iaijutsu vocabulary, where it names the whole art of drawing, cutting, and re-sheathing a sword in one unbroken motion — the fight is decided in that single flash, before an opponent even sees the blade move. Zenitsu's version leans into this literally: he fights unconscious, sword undrawn until the last possible instant, so the panel where 霹靂一閃 finally fires reads as a thunderclap arriving before its lightning — sound and light collapsed into one frame. VIZ's "Thunderclap and Flash" preserves that same sound/light pairing as a hendiadys rather than picking one half to translate, which keeps the SFX-style lettering rhythm of the original bubbles intact.
Within Thunder Breathing's own naming logic, 霹靂一閃 is the plainest, most elemental entry: every other form (稲魂 "Rice Spirit," 聚蚊成雷 "Thunder Swarm," 遠雷 "Distant Thunder," 熱界雷 "Heat Lightning," 電轟雷轟 "Rumble and Flash") is a multi-hit flourish, while the First Form is just the sound and the flash — a thunderbolt with no further embellishment. That it's also the only form Zenitsu can access is not incidental: his grandfather Jigoro tells him outright, mid-training, that if you can only do one thing, "do it superbly… polish it to the utmost of the utmost," comparing him to a blade forged by being struck over and over until every impurity is gone. Kaigaku, meanwhile, masters every other form (2 through 6) yet — even after becoming a demon — can never perform this one; talent without the foundational form turns out to be its own kind of incompleteness.
Three named variants ride on the base technique across the series: 六連 ("Sixfold," first landed against the drum demon Kaigyo's brother in vol. 4), 八連 ("Eightfold," unleashed against Rui's mother in the Spider Mountain arc), and 神速 — literally "godlike speed," rendered by VIZ as both "GOD-SPEED!" (vol. 23, p. 28) and "GODSPEED!" (vol. 23, p. 38) within ten pages of each other, a small but real hyphenation inconsistency inside VIZ's own production. Zenitsu narrates godspeed as a technique he can only fire twice before his legs give out — each use quite literally billed as spent lifespan, first to escape rubble, then for one final strike, turning a flashy damage-multiplier into a countdown clock the reader can track alongside him.