When the base letter descends past thebaseline, the spacing diacritic⟨˒⟩ may be used instead.
The IPA explains the symbol as follows: "Lip position in vowels is regarded as forming a continuum from close rounded (as for Cardinal [u]) to spread (as for Cardinal [i]). [...] the 'over-rounding' diacritic may be used to indicate a value further from 'spread' than entailed by the reference value of a symbol, for instance[ɯ̹] for a close back vowel [with] only a little spreading, instead of completely spread as would be normal for that vowel."[1]
