Curry is the filling that makes the sealed pocket essential rather than incidental. The curry hot sando takes the pressed-sandwich format from the baseline hot sando and fills it with Japanese curry, the thick, mellow, slightly sweet brown sauce that is its own pillar of the national table. A loose curry inside an open sandwich would be a disaster of dripping and collapse. Inside the clamped iron it becomes the ideal filling: the crimped border acts as a dam, the bread toasts into a rigid shell, and what would otherwise run everywhere is held in a hot, contained pocket that only releases when you cut it.
The whole craft is about controlling moisture. Japanese curry is already engineered to be thick, and for this use it wants to be on the drier, more reduced side so it does not turn the crumb to paste or burst the seal under pressure. The cook spreads it short of the crusts, often with a layer of cheese, since melted cheese binds the curry and gives the filling enough body to hold a clean cross-section. The press then has to weld the edges firmly, because curry under heat expands and looks for the weakest corner. A good curry hot sando cuts to reveal a glossy, steaming, cohesive filling that stays where the knife left it, framed by a deeply toasted shell that contrasts the soft spiced interior. A poor one fails in one of two predictable ways: a watery curry that soaks the bread into a soggy gray slab, or a blown seal that vents the filling onto the plate and leaves a sad hollow parcel. The interior must be hot all the way through, which a thick curry resists, so the press time runs a little longer than it does for a quick ham-and-cheese.
This sits at the intersection of two Japanese staples, curry and the pressed sandwich, and the variations follow the curry world. People build it with katsu curry by adding a thin cutlet, with extra cheese for a richer melt, or with a hotter roux for people who want the spice to push back against the sweet bread. The katsu curry build in particular, with its fried cutlet changing the texture entirely, is enough of a distinct experience that it deserves its own article rather than being crowded in here.