Matsya Purana — Śrāddha Procedure: Types
यज्ञोपवीती निर्वर्त्य ततः पर्युक्षणादिकम् प्राचीनावीतिना कार्यम् अतः सर्वं विजानता //
yajñopavītī nirvartya tataḥ paryukṣaṇādikam prācīnāvītinā kāryam ataḥ sarvaṃ vijānatā //
Having arranged the sacred thread in the yajñopavīta position, one should then perform acts such as sprinkling (paryukṣaṇa) and the rest; but these are to be done with the thread in the prācīnāvīta position—so understands the one who knows the entire procedure.
This verse is not about pralaya; it focuses on correct ritual procedure—specifically the proper wearing of the sacred thread during different steps of a rite.
It reinforces the dharmic duty of performing rites with procedural accuracy; a householder (and by extension a king who upholds dharma) must observe correct yajñopavīta/prācīnāvīta conventions during sanctifying actions like paryukṣaṇa.
The ritual significance is the regulated shift in sacred-thread position: begin in yajñopavīta, then perform paryukṣaṇa and related acts in prācīnāvīta—an important procedural marker for correctness and purity in worship/rites.