सोपानम् ५: आभ्यन्तर-प्रयत्नः (The 5 Internal Efforts)
Sopāna 5: Vocal Contact Mechanics — Spṛṣṭa, Īṣat-spṛṣṭa, Īṣad-vivṛta, Vivṛta, and the Mystery of Saṃvṛta (8.4.68 "a a")
१. दर्शनम् — Intra-Oral Contact Dynamics
"यत्नो द्विधा — आभ्यन्तरो बाह्यश्च।
आद्यः पञ्चधा — स्पृष्ट-ईषत्स्पृष्ट-ईषद्विवृत-विवृत-संवृतभेदात्॥"
yatno dvidhā — ābhyantaro bāhyaśca |
ādyaḥ pañcadhā — spṛṣṭa-īṣatspṛṣṭa-īṣadvivṛta-vivṛta-saṃvṛtabhedāt ||
— वरदराजः, लघुसिद्धान्तकौमुदी (संज्ञा-प्रकरणम्)
"Effort (Prayatna) is twofold: Internal (Ābhyantara) and External (Bāhya). The primary internal effort before sound emerges is of 5 kinds."
While Sthāna (स्थानम्) defines WHERE the sound originates, Ābhyantara Prayatna (आभ्यन्तर-प्रयत्नः) defines HOW the tongue and vocal organs touch that locus inside the mouth cavity prior to sound release.
In Pāṇinian phonetics, this intra-oral action determines whether two sounds can be considered homogeneous (सवर्णम्) for Sandhi substitutions and operations.