線香花火 (せんこうはなび, literally “incense fireworks”) is a type of Japanese fireworks that usually gets translated into English as “Japanese sparklers.” They are slightly different from the usual American variety of sparklers in that you hold them facing down toward the ground rather than up toward the sky. Once lit, a small ball of what can only be described as “magical molten fire stuff” creeps up the thin thread of the sparkler, sending out random flashes of fractal-like sparkles. They are incredibly hypnotic to watch. (See photo at the Japanese Wikipedia site or in this video with music from Amano Shigeru.)
Once the ball gets to a certain point, it fizzles out quickly, which gives it the idiomatic meaning “flash in the pan” or “to fizzle out quickly.” You often see 線香花火のよう(に・な) and sometimes 線香花火的.
Belated Happy 4th of July!
Update: Commenter robert found this great video showing exactly how they sparkle. Looks like there are more videos searching under “senkou hanabi” than “線香花火”.