Rule. Wherever a rule prescribes guṇa or vṛddhi by using those very words, the word ikaḥ, "of ik", presents itself in that rule. The guṇa or vṛddhi therefore comes in place of an ik vowel (i, u, ṛ, ḷ) of the item concerned. Bhaṭṭoji's word for "presents itself" is upatiṣṭhate: the genitive ikaḥ becomes part of the other rule's sentence, and each rule construes it in its own way. The words guṇa and vṛddhi are carried into this sūtra by anuvṛtti from SK 16 and SK 17.
Example.
mid (a root) → the rule mider guṇaḥ (A 7.3.82) says "guṇa of mid", naming no sound → ikaḥ presents itself: the guṇa falls on the ik of mid, its i → the i is replaced by its guṇa, e (why e and not a is settled at SK 39) → med-.
Likewise the rule sārvadhātukārdhadhātukayoḥ (A 7.3.84), which prescribes guṇa for a root before certain affixes: its working on the i of ci and the u of stu is shown at SK 39 (cetā, stotā).
Applies when / blocked by. Only where the words guṇa or vṛddhi actually prescribe something. Not at a mere restatement: vṛddhir yasyācām ādis tad vṛddham (A 1.1.73) uses the word vṛddhi to describe a word, not to prescribe a change, and a paribhāṣā does not present itself at a restatement. Not where the words are absent: tyadādīnām aḥ (A 7.2.102) prescribes an a without calling it guṇa, so ikaḥ does not come; which sound that a replaces is settled by SK 42 instead.
Watch out. The sūtra's ikaḥ stands for its own form. What is inserted into the other rules is that genitive word itself, so read "guṇa of mid" as "guṇa of the ik of mid".
Where guṇa and vṛddhi are prescribed by the words "guṇa" and "vṛddhi," there the word ikaḥ, ending in the sixth [genitive] case, presents itself (upatiṣṭhate).
iko guṇavṛddhī — iko guṇavṛddhī. [In Bhaṭṭoji's gloss] "ika iti": the word [ikaḥ], which [in the sūtra] ends in the sixth case, here refers to its own form (svarūpa-para), is neuter in gender, and ends in the first-case singular; since [its ending] su has been elided by luk, the lengthening of an at/as-ending [stem before su, prescribed] by "atvasantasya [cādhātoḥ]" (A 6.4.14), does not take place. The meaning is: the word is *ikas* [the genitive form quoted as a word, ending in -as]. "upatiṣṭhate" [presents itself] is [the predicate] to be supplied. From "vṛddhir ādaic" (A 1.1.1) and "ad eṅ guṇaḥ" (A 1.1.2), [the words] vṛddhi and guṇa are carried forward (anuvṛtti) [into this rule]. The word "iti" must be supplied (adhyāhārya); "where they are prescribed, there" must likewise be supplied. The construal (yojanā) is: where guṇa and vṛddhi are prescribed by uttering [the words] "guṇa" and "vṛddhi," there the sixth-case-ending word ikaḥ presents itself. This is just what he states with "guṇavṛddhiśabdābhyām" etc. "upatiṣṭhate" — the meaning is: it becomes [syntactically] connected (saṅgata). The ātmanepada is by "upād devapūjāsaṅgatikaraṇa-" [the rule prescribing ātmanepada for upa-sthā in the senses of worship of gods, joining together, etc.]. This position of the presentation of the word (padopasthiti-pakṣa) is the one established as siddhānta in the Bhāṣya and elsewhere. "sārvadhātukārdhadhātukayoḥ" (A 7.3.84) and "mider guṇaḥ" (A 7.3.82) are examples. The manner in which "ikaḥ" construes [in each rule] will become clear in the several places. By the statement "where they are prescribed (vidhīyete)": in a mere restatement (anuvāda) such as "vṛddhir yasyācām [ādis tad vṛddham]" (A 1.1.73), "ikaḥ" does not present itself, because a paribhāṣā does not present itself at a restatement. In "tyadādīnām aḥ" (A 7.2.102) and the like, too, this does not present itself, since the words "guṇa" and "vṛddhi" are not heard there.
- textual The digital source reads 'ṣaṣṭanta' twice for 'ṣaṣṭhyanta', 'spaṣṭībhavaṣyati' for 'spaṣṭībhaviṣyati', and a stray 'o' after 'dīrgho na'. The transmitted 'ikasśabda' is retained as coherent: the svarūpa-para word is the genitive form ikas quoted as a word, which is why A 6.4.14 is cited as not applying.