For the penultimate o of the guṇa-ated aṅga (stem) goh (from guh) there is the substituted ū before an affix beginning with a vowel.,
Thus निगूहति, निगूहकः, साधुनिगूहिन्, निगूहम्, निगूहन्ति, गूहो वर्तते ॥ Why do we say 'penultimate'? So that the substitution should not apply any where else. The form गोह् is taken in the sutra to prohibit the application of the rule to cases where गुह् does not assume the form गोह् ॥ Therefore not here, निजुगुहतुः, निजुगुहुः ॥ This change will not take place before the affixes beginning with a consonant; as निगोढा, निगोढुम् ॥ Some say, the word is exhibited as गोह in order to prohibit the अय् substitution of णि before ल्यप् ॥ As निगाहि + ल्यप् = निगूह्य (6.4.56 not applied), the ऊ substitution being considered as asiddha 6.4.22 would have brought in अय्, गूह् being considered as a word having a light vowel for its penultimate. According to Kasika the ऊ substitution, however, is not asiddha, as their places of operation are different.,