Rule. After a neuter aṅga ending in short a, the endings su and am are both replaced by am. This is an apavāda (an exception) to SK 319: where SK 319 would delete the ending, SK 309 keeps an am instead. Replacing am by am is not idle; it is exactly what saves the accusative ending from luk.
Example.
jñāna + su → su → am → jñāna + am
jñāna + am → a + a → the single vowel a by ami pūrvaḥ (SK 194; the pūrvarūpa, "the shape of the former vowel") → jñānam (nominative singular).
jñāna + am (accusative) → the same steps → jñānam (accusative singular).
Vocative singular: he jñāna + su → su → am → pūrvarūpa → he jñānam → by eṅ hrasvāt (SK 193) the consonant of the vocative ending is dropped → he jñāna.
Applies when / blocked by. Only after a stem in short a. "After" is read by tadanta-vidhi: "after a" means "after a stem ending in a", because the heading aṅgasya is here turned into an ablative and qualified by ataḥ.
Watch out. Why is the substitute am and not just m? Bhaṭṭoji's commentators explain: if a bare m replaced su, that m would count as a consonant-initial case-ending, and supi ca (A 7.3.102) would wrongly lengthen the stem to jñānām. So the substitute is the full syllable am, which then merges with the stem-final a by pūrvarūpa. In the vocative, what eṅ hrasvāt removes is only the m left over after pūrvarūpa; scholars discuss exactly how that residue still counts as the vocative ending.
After an a-ending (ataḥ) aṅga that is neuter (klība), am is substituted for su and am (svamoḥ — the nominative and accusative singular). By "ami pūrvaḥ" (SK 194) there is pūrvarūpa (the single substitute in the shape of the preceding vowel): jñānam. By "eṅ hrasvāt [saṃbuddheḥ]" (SK 193) there is elision of the consonant alone (hal-mātra-lopa): he jñāna (vocative singular).
ato'm — After the word jñāna there is su; and when the luk-elision of that [su] by "svamor napuṃsakāt" [7.1.23] is obtained, [the present rule] "ato'm" [applies]. "ata" is in the ablative (pañcamī). The governing (adhikṛta) "aṅgasya" [6.4.1, standing in the genitive] is transformed into the ablative and is qualified by "ata"; thereby there is a tadanta-vidhi (the rule applies to what ends in a). "svamor napuṃsakāt" [7.1.23] is carried forward (anuvṛtti). He states this by "ato'ṅgāt": the meaning is "from an a-ending (adanta) aṅga." "jñānam" — when am is substituted for su, by "ami [pūrvaḥ]" there is pūrvarūpa; this is the point. But the prescription of am for the am (accusative) affix is for the purpose of nullifying the luk [that "svamor napuṃsakāt" would otherwise cause].
Objection: Let the division [of the sūtra] be simply "ato m" [taking m, not am, as the substitute]; for with m-kāra as the substitute for su, "jñānam" is established; and in the am (accusative), by "ādeḥ parasya" [1.1.54] the m substitutes the initial a-kāra of the affix, and then by saṃyogānta-lopa of the final m-kāra "jñānam" is likewise established. Response: Not so. For if it were so, in "jñānam" the undesired long vowel (dīrgha) would arise by "supi ca" [7.3.102, since the mere consonant m is a sup beginning with yañ]. And it should not be argued: "The m-substitute, which operates in dependence on the collocation (saṃnipāta) of the a-ending [aṅga], cannot become the cause (nimitta) of the long vowel that destroys that very collocation, because this conflicts with the saṃnipāta-paribhāṣā [nimittam atadvighātakam]" — because it has [already] been stated that, when the dīrgha by "supi ca" is to be effected, the saṃnipāta-paribhāṣā does not operate. Enough of this.
"he jñāna" — When "he jñāna-s" stands, once am is substituted for su and pūrvarūpa is done, by "eṅ hrasvāt" there is elision of the m-kāra; the form is "he jñāna." Objection: In "eṅ hrasvāt [saṃbuddheḥ]" it is proper that the [aṅga] implied by "saṃbuddhi" construe precisely with the saṃbuddhi; and thus what is obtained is "elision of the hal that is a member of the saṃbuddhi which follows an aṅga ending in eṅ or in a short vowel (hrasva)." And thus, in the present case, once pūrvarūpa is done — if that pūrvarūpa be taken as belonging to the end of the preceding (pūrvānta) — then "jñāna" is an a-ending aṅga, and there is no saṃbuddhi following it; for the mere m-kāra is not the saṃbuddhi (the saṃbuddhi being properly the "am" that stands in place of su), and because the maxim "ekadeśavikṛtam ananyavat" (a thing altered in one part is treated as not other) does not apply to a half-alteration. Nor should it be argued, "Why should not 'eṅ hrasvāt' operate before pūrvarūpa?" — because by paratva pūrvarūpa alone operates first. Nor should it be argued: "Since pūrvarūpa, being the end of the preceding, is included in the aṅga, 'jñāna' is an a-ending aṅga; and since 'am' is the initial-following (parādi), it is the saṃbuddhi — depending on both these, the elision by 'eṅ hrasvāt' of the hal-member of it, i.e. of the mere m-kāra, is unobstructed" — because such a double dependence is prohibited by "nāntādivat" [one and the same element cannot be treated as belonging both to the end of the preceding and to the beginning of the following]. Hence he says "eṅ hrasvāt: there is elision of just the consonant (hal-mātra-lopa)." The meaning is: once pūrvarūpa is done, the elision by "eṅ hrasvāt" is of the mere m-kāra that remains as the residue of the saṃbuddhi. In accordance with attested usage (lakṣya), the aṅga implied by "saṃbuddhi" does not construe with the saṃbuddhi [as a whole], but rather construes precisely with the hal that is a member of the saṃbuddhi. And thus what is obtained is "elision of the hal-member of the saṃbuddhi that follows an aṅga ending in eṅ or in a short vowel." And in the present case, even after pūrvarūpa is done, the mere m-kāra, being a member of the saṃbuddhi, does indeed follow an aṅga ending in a short vowel; hence its elision by "eṅ hrasvāt" is unobstructed. The full treatment is elsewhere.