If I had to guess, Reimu saw the sparkler, declared the little droplet of burning whatever-it-is as a sweet-sweet, and caught the thousand degree droplet when it fell.
(Japanese sparklers apparently differ from the other style, at least those here in the US. In the US, sparklers are all solid, whereas in Japan their sparklers have a little droplet of liquid which provides the sparks as it consumes the rest; losing the droplet is like losing the cherry on a cigarette.)
1: Yuyu! Mishter! What iz dat? Can it chake it eajy?
It's a sparkly mister shiny flower!! It's very pretty! It can really take it easy!!!
2: *SPARK SPARK*
Yu~n! Ichs vewy pwetty! It can chake it eajy!
3: Ah! Mister flower's very sweet sweet nectar is about to fall out!
*spark spark*(lesser sparking sound anyway)
Yuyu!!! Mishter sweet sweet! Fall into Remu's mouth eajy!
Rip-!! Ripp!!
4: *appear*
There's still a lot! Eat them all easy!
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I'm pretty sure biribiri is the sound of him ripping off some tape and in the 4th panel, he tapes the yukkuri down to assist it in getting more of the "sweet sweet nectar"