The junction periods are easy to read past in the Puranic arithmetic, a bookkeeping detail wedged between grander numbers, but they carry a distinct idea: ages do not begin and end, they dawn and fade. Each yuga is preceded by a sandhya and followed by a sandhyamsa, each a tenth of the age it borders, so fully a sixth of all cosmic time is spent in transition.
Manu's figures make the pattern plain: the Krita age of 4,000 divine years takes a 400-year dawn and a 400-year twilight; the Kali age of 1,000 divine years is wrapped in junctions of 100 each. The scheme has practical consequences inside the tradition's own storytelling. Events that seem to belong to the wrong age, virtue flourishing after the Kali Yuga has begun, or corruption appearing before the Dvapara ends, are placed in the junctions, where the qualities of two ages overlap and interpenetrate. The twilight of one age is already colored by the next; the dawn of an age still carries its predecessor's residue. Commentators have used the device to soften the harshest reading of yuga determinism: even deep in a dark age, the light of the previous one has not wholly gone, and the next dawn is already, in some measure, present. Time in this cosmology is not a staircase but a gradient, and the sandhya is the doctrine that says so.