Verse 2.4.1

सर्वास्वप्सु पञ्चविधंसामोपासीत मेघो यत्सम्प्लवते स हिंकारो यद्वर्षति स प्रस्तावो याः प्राच्यः स्यन्दन्ते स उद्गीथो याः प्रतीच्यः स प्रतिहारः समुद्रो निधनम् ॥ २.४.१ ॥

sarvāsvapsu pañcavidhaṃsāmopāsīta megho yatsamplavate sa hiṃkāro yadvarṣati sa prastāvo yāḥ prācyaḥ syandante sa udgītho yāḥ pratīcyaḥ sa pratihāraḥ samudro nidhanam || 2.4.1 ||

1. One can perform the fivefold Sāma worship in all kinds of water. The coming together of scattered clouds is hiṃkāra. That which pours forth rain is the prastāva. The udgītha is that [river] running eastward, and that which runs westward is the pratihāra. The sea is the nidhana.

Word-for-word explanation:

Sarvāsu apsu, in all kinds of water; pañcavidham sāma upāsīta, a person should perform the fivefold Sāma worship; meghaḥ yat saṃplavate, the clouds which join together and consolidate; saḥ hiṃkāraḥ, that is hiṃkāra; yat varṣati, that which pours rain; saḥ prastāvaḥ, that is the prastāva; yāḥ prācyaḥ syandante, that [water, or river] which goes eastward; saḥ udgīthaḥ, that is the udgītha; yāḥ pratīcyaḥ, that which goes westward; saḥ pratihāraḥ, that is the pratihāra; samudraḥ nidhanam, the sea is the nidhana.

Commentary:

The fivefold Sāma worship can be performed in all forms of water. For instance, when clouds come together to produce rain, that can be thought of as hiṃkāra, because the syllable hiṃ marks the beginning. And when rain starts falling, that is thought of as the prastāva, because the rain is ‘ready’ to scatter in all directions. (This ‘readiness’ is prastāva.) The water, or river, that flows eastward (the Ganges, for instance) is called the udgītha, because both represent excellence. The water flowing westward (pratīcya) is the pratihāra, because of the prefix prāti being common to both. The sea is the nidhana, because when the water flows into the sea it loses its separate identity, which is ‘death’ (nidhana).