Despite its fearsome sounding name, the Happo Daikarin is actually something of a joke. It involves Happosai hurling home-made bombs at a target, engulfing them in explosions. Despite this, they are actually extremely powerful; Happosai has been seen accomplishing feats like knocking out Pantyhose Taro in monster form with just one of these bombs.
Happosai occasionally displays the ability to rapidly launch barrages of bombs, hurling them with great speed to cover an area with multiple explosions. He also sometimes uses larger than normal bombs, with correspondingly more powerful explosions. Certain depictions and imagery would suggest that the Daikarin technique is augmented by the user's fighting spirit.
History of the Technique
It's unknown when or how Happosai developed this move.
Happosai does not initially use this technique until some time after he has joined the regular cast. It first appears in its own distinctive storyline; the manga's Happosai's Ultimate Technique Arc and the anime's Rub-a-Dub-Dub! There's a Pervert in the Tub.
In both versions of the storyline, we learn from Soun Tendo and Genma Saotome that they saw Happosai use this technique only once, to destroy a boulder that would have flattened him whilst he was stealing a bra. The explosion knocked them out cold, as they were riding the boulder at the time, so they never saw what the move entailed. When they came to, Happosai was mourning over the fact he'd accidentally burned up his bra in the process. Stricken with grief, he put the technique out of mind and buried the Happo-Fire Burst Scroll in a secret location
The rest of the storyline involves Happosai's attempts to recover the scroll; although it is ultimately destroyed, Ranma Saotome unwittingly jogs Happosai's memory and allows him to recover it. He promptly makes it his signature attack for the rest of the series.
In the anime-only storyline The Horrible Happo Mold-Burst! a weakness of the Happo Daikarin is revealed: because it requires paper-based bombs, when the weather is especially wet, they tend to get too damp to ignite. Consequently, Happosai creates a variant of the technique that deploys gray bombs filled with poisonous, paralytic clouds of fast-growing mold. It only appears in the one episode, and is not mentioned again for the rest of the series.
In this episode, Happosai names a rapid-fire variant of the Happo Mold-Burst as the Mega Mold-Burst.