Recipes for Indian sweets rarely give a syrup temperature. They describe threads formed between finger and thumb, which is a working measure of how much water the syrup still holds.
What the thread test is measuring
As a sugar syrup boils, water leaves and the sugar concentration climbs. Higher concentration means higher viscosity and a higher boiling temperature.
Lifting a drop and pulling it apart shows how far this has gone. A syrup that will not thread is still dilute, one thread means further along, two threads further still.
The test works because viscosity and concentration move together, so a physical property stands in for a measurement the kitchen has no instrument for.
Why each stage produces a different sweet
A syrup at the loose end soaks into a porous fried sweet and keeps it moist without setting, which is what a soft, syrup-filled sweet requires.
A little further and the syrup will cling and set softly around a dough, giving a coating that stays tender rather than running off.
Further still and the sugar crystallises as it cools, producing the granular, brittle texture of the crystalline sweets.
How cooks control crystallisation
Sugar crystallises readily once concentrated, and stray crystals on the side of the pan will seed the whole batch. Cooks wash the sides down or cover the pan briefly to dissolve them.
Adding a small quantity of acid or a different sugar interferes with crystal formation by giving the molecules something that will not fit the lattice.
Where crystals are wanted, the opposite is done, and the mixture is stirred vigorously as it cools to produce many small crystals rather than a few large ones.
Why the sweet's own state matters
A hot sweet dropped into hot syrup absorbs quickly, while a cooled one absorbs slowly and unevenly. The temperature difference between the two governs how deep the syrup travels.
Fried sweets are especially sensitive, since the internal structure is full of air pockets that fill or fail to fill depending on the moment of contact.
This is why recipes specify the state of both components rather than only the syrup, and why timing is treated as part of the method.
What goes wrong and why
A syrup taken too far leaves the sweet dry and hard, because the sugar sets before it has penetrated and then draws moisture outward as it cools.
Undercooked syrup does the reverse, leaving the sweet sitting in liquid that never sets and slowly turning it soggy.
Neither fault can be corrected afterwards, since the syrup and the sweet have already reached their final relationship by the time the mixture cools.