पुगन्तलघूपधस्य च

गुण is substituted before a सार्वधातुक and an आर्धधातुक affix for the इक् vowels of the Causative अङ्ग (stems) which take the augment प् -- अर्त्तिह्रीब्लीरीक्नूयीक्ष्माय्यातां पुङ्णौ 7.3.36, and for the short penultimate vowel of a root which ends in a single consonant.
Karika:-

संयोगे गुरुसंज्ञायां गुणो भेत्तुर्न सिध्यति ।
विध्यपेक्षं लघोश्चासौ कथं कुण्डिर्न दुष्यति ॥
धातोर्हिनुम कथं रञ्जे, स्यदिश्रथ्योर्निपातनात् ।
अनङ्लोपशिदीर्घत्वे विध्यपेक्षे न सिध्यतः ॥
अभ्यस्तस्य यदाहाचि लङर्थं तत्कृतं भवेत् ।
क्नुसनो यत्कृतं कित्त्वं ज्ञापकं स्याल्लघोर्गुणे ॥

As व्लेपयति, ह्रेपयति, क्रोपयति, भेदनम्, छेदनम्, भेत्ता, छेत्ता ॥ Of course the vowel should be laghu or light, before the addition of the affix; the heaviness caused by the addition of the affix, will not prevent. Thus भिद् + तृ, though द् + तृ = त्तृ causes the इ to become heavy, that will not prevent guna: for विध्यपेक्षं लघुग्रहणम् ॥ Objection : If this be so, why the forms कुण्डितृ, हुण्डितृ are not incorrect, for in them also the vowel is laghu (the roots are कुड् and हुड्), and the augment म् is added afterwards by a Vidhi rule? Answer: The augment म् is added to the root, and becomes upadesivat. (See 7.1.58). Objection: If so, how do you cause Vriddhi in रञ्ज्, as in रागः with घञ्; as रञ्ज् + घञ् = रज् + घञ् (the ञ् being dropped by 6.4.27), for it is after the elision of ञ्, that the of रज् becomes penultimate and can admit of Vriddhi by 7.2.116? Answer: The exceptional forms स्यदः from स्यन्द् + घञ्, and श्रथः from श्रन्थ + घञ्, taught in (Vl. 4.28); 6.4.29 teach by implication that roots of this form take Vriddhi as a general rule. Objection: If the विध्यपेक्ष maxim is not of universal application: though by 6.4.134, the अ of अन् &c. is elided in bha stems, like राजन् thus राज्ञा; yet the अ shoud not be elided if you be consistent, in आनङ् augment added by 7.1.75 to दधि, अस्थि &c. In fact, you could not get the forms दध्ना सक्थ्ना &c. Moreover though there can be the lengthening of the penultimate in सामन् = सामानि (before शि affix Nominative Plural), yet not in the case of कुण्डानि from कुण्ड for here न् is added by another rule 7.1.72. If you say, the maxim is not of universal application, then there can be no guna of इ in भिद् to form भेत्तृ ॥ Answer: The guna takes place in forms like भेत्तृ &c, because the prohibition of the following rule 7.3.87, with regard to the affixes beginning with a vowel, proves by implication that before affixes beginning with a consonant, as तृ, the guna also takes place. Objection: The prohibition in the case of vowel beginning affixes is for the sake of लङ्, to form अनेनेक् ॥ This is derived from निजिर् (Juhotyadi 11), in the Imperfect, as अ + निज् + शप्श्लु + तिप् then reduplication 6.1.10, then guna of the reduplicate, 7.4.75, then तिप् which had become त in the Imperfect, is elided by 6.1.68. Thus अनिज् + श्लु + त् = अ + निनिज् + त् 6.1.10 = अ नेनिज् + त् 7.4.75 = अ नेनिज् 6.1.98 = अननक्. The elided त् produces its effect, the guna by 7.3.86. This is why अच् is taken in sutra 7.3.87, namely अजादि affixes do not cause guna, the हलादि affixes like त् cause guna. You cannot therefore say that अचि in 7.3.87 is a jnapaka. Answer: The sutra त्रसिगृधिधृषिक्षिपेः क्नुः 3.2.140 teaches the addition of नु to त्रस् &c, as गृध्नुः; if this नु had not tended to cause the guna of the penultimate vowels of these roots, what was the necessity of making this affix a कित्? Similarly 1.2.10 teaches that सन् (Desiderative) is कित् after roots ending in consonants. These rules show that words like भत &c get guna, and the final consonant of the root plus the initial consonant of the affix, does not make the root vowel heavy. Objection: The कित् of सन् is for the sake of the elision of the nasal, in धिप्सति, धाप्सति 6.4.24. Answer: Let it be so: still the कित् of क्नु is enough for us.

The "upadha short" must be the vowels of the इक् pratyahara. Therefore in भिनत्ति, the penultimate is short अ (of श्नम्) and it does not take guna. In fact, the word penultimate qualifies the word इक् understood. Others explain it by saying that पुगन्त is to be analysed by पुकि अन्त "in the vicinity of प्" ॥ The word अन्त means समीप, and means the इक् vowel in the proximity of पुक् ॥ The word लघूपधा should be analysed as लघ्वी उपधा and is a Karmadharaya compound, and means "a short or light penultimate". The word पुगन्तलघूपध is a Samahara Dvandva.