A pani puri shell is a hollow sphere with walls thin enough to see light through. The shape is produced by steam, and it survives because the dough sets around it.
Steam is what inflates the disc
Water in the dough turns to vapor almost instantly in hot oil, expanding to occupy far more space than the liquid did. That expansion pushes the two surfaces apart.
The disc has to be thin and evenly rolled for this to work, since a thick spot resists the pressure and anchors the dough, leaving a shell that only half inflates.
Vendors roll a large sheet and cut discs from it rather than rolling each one, precisely because uniform thickness across the batch is what determines the puff rate.
The crust has to set before the steam leaves
Oil at frying temperature dries and hardens the outer surface quickly. If setting happens fast enough, the shell holds its inflated shape once the internal vapor cools.
Oil that is too cool loses the race. The dough absorbs fat, softens, and the disc collapses into a greasy flat crisp instead of a sphere.
Cooks judge the temperature by dropping in a scrap and watching how quickly it rises and colors, since a thermometer in shallow oil reads the pan more than the frying zone.
The flour choice controls the wall
Semolina gives a rigid, brittle wall that stays crisp when filled, because its coarse particles form a structure that does not soften quickly under liquid.
Wheat flour alone produces a more pliable shell that puffs readily but goes soggy faster once the spiced water goes in.
Most recipes blend the two, trading some ease of rolling for a wall that survives the few seconds between filling and eating, which is the whole point of the format.
Resting the dough matters
A stiff, well-rested dough rolls thin without tearing and without springing back. Under-rested dough contracts as it is cut, thickening the disc and reducing the puff.
The dough is deliberately drier than most, since excess water makes it slack, and a slack disc stretches rather than holding the tension the steam needs to push against.
Storage is the vendor's real constraint
Fried shells are only crisp while they are dry, and they draw moisture from humid air. Vendors keep them sealed and pull them out in small quantities.
This is also why a bag bought from a store can disappoint. A shell that has softened will not crisp back, since the structure has already relaxed.
Reheating briefly in a dry oven drives off absorbed moisture and restores some of the snap, which is the closest a home kitchen gets to a fresh batch.