This rule reverses the Ilkorin a-affection in some cases, so that primitive [i] and [u] remained (or were restored) despite the presence of a final [a] in a word. Some clear examples of this rule are ᴹ✶lindā > Ilk. lind (Ety/LIND) and ᴹ✶tungā > Ilk. tung (Ety/TUG). In all the obvious examples of the rule, the restored vowel precedes an [nn], [nd] or [ŋg].

Sound changes without a-affection: It is unclear whether the [i], [u] are restored by this rule, or are simply prevented from undergoing a-affection in the first place. There is at least one example of this change that does not involve a-affection, however: ᴹ✶kwentrō > Dor. cwindor had an original primitive [e] that developed into [i] before [nd] (Ety/KWET). This lends weight to the idea that this rule was a restoration of the sound rather than a preservation. Another possible example of a pure vowel change is Dor. nîw < [niŋgwe < neŋgwe <] ᴹ✶neñwi. Both these examples were suggested by Helge Fauskanger (AL-Doriathrin/cwindor, nîw).

The vowel change did not occur with ᴹ✶kwenedē > Dor. cwend and ᴹ✶besnō > Ilk. benn, so probably this phonetic change occurred before the Ilkorin syncope and before [sn] became [nn].

Developments before other nasals: For the combination [mb], it appears that this restoration did not occur: consider ᴹ✶dimbā > Ilk. dem (Ety/DEM). It seems that Tolkien was also not entirely sure of this development: an earlier version of the entry for the root ᴹ√DEM had ᴹ✶dembē > Ilk. dim (EtyAC/DEM), where it seems that [-emb-] did become [-imb-]. Variations in the adjective suffix -en vs. -in (from ᴹ✶-inā) may indicate that Tolkien also considered applying this rule for simply nasal [n] instead of just [nn] and [nd].