This is a minor rule applying to one verbal suffix. In its simple form, the suffix is /-ni/ giving the Past2 form of certain high-frequency, mostly monosyllabic verbs: /=na-ni/ 'saw', /=ngu-ni/ 'ate', /=yanga-ni/ 'heard', /=i-ni/ 'gave'. The corresponding Nonpast2 suffix for these verbs is /-yi:/ as in /=na-yi:/ 'sees'. These suffixes are not characteristic of other verb classes.
However, there is one other fairly small verb class which does have a closely associated suffixal paradigm. This is the N class (see Table 11-4, in Chapter 11). The Nonpast2 for this class is /-nji:/ which we analyse as //-n-yi:/ ith //y// → /j/ by Hardening P-18. In other words, this is the same /-yi:/ ending seen in /-na-yi:/ 'sees', along with a class marker /-n-/ Now the Past2 form, where we might expect //-n-ni// reducing by Geminate Contraction P-31 to /-Ø-ni/ is in fact /-n-di/ for the N class. Thus /=yara-n-ji:/ 'smells', Past2 /-yara-n-di/ 'smelled'. If we assume that /-n-di/ is structurally the /-n-/ class marker plus the same Past2 element seen in /=na-ni/ 'saw', we need a minor rule changing //n// to /d/ by dissimilation to the preceding nasal consonant. This is not a productive rule; other combinations of two //n// segments are reduced to /n/ by Geminate Contraction P-31.