[1] That is, the airstream is controlled by moving the glottis downward in addition to expelling air from the lungs.
In the International Phonetic Alphabet, implosives are indicated by modifying the top of a letter (voiced stop) with a rightward-facing hook: ⟨ɓ ɗ ᶑ ʄ ɠ ʛ⟩.
During the occlusion of the stop, pulling the glottis downward rarefies the air in the vocal tract.
Because the airflow required for voicing reduces the vacuum being created in the mouth, implosives are easiest to make with a large oral cavity.
The IPA once dedicated symbols ⟨ƥ ƭ 𝼉 ƈ ƙ ʠ ƙ͜ƥ⟩ to such sounds.
The IPA had also suggested the possibility of a superscript left pointer, ⟨p˂ t˂ ʈ˂ c˂ k˂ q˂ k͡p˂⟩, which might also be used for fricatives, but it was not approved by the membership.
They appear to be entirely absent as phonemes from Europe and northern Asia and from Australia, even from the Australian ceremonial language Damin, which uses every other possible airstream mechanism besides percussives.
However, Alpher (1977) reports that the Nhangu language of Australia may actually contain implosives, though more research is needed to determine the true nature of these sounds.
[13] Fully voiced stops are slightly implosive in a number of other languages, but this is not often described explicitly if there is no contrast with modal-voiced plosives.
Sindhi and Saraiki have an unusually large number of contrastive implosives, with /ɓ ᶑ ʄ ɠ/.