When the base letter descends past the baseline, 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 'under-rounding' diacritic may be used to indicate a value on that continuum further from 'close rounded' than entailed by the reference value of a symbol, for instance [u̜] for a close back vowel with little rounding, and [ɛ̜] for a vowel with more spreading than the open-mid reference vowel."[1]
