The phonetics of implosives
Voiceless implosives are sounds which are made on a laryngeal (glottal) ingressive airstream mechanism. The vocal folds in the larynx are firmly pressed together (the glottis is closed) and the larynx as a whole is pulled downwards by means of the infra hyoid muscles. This creates negative pressure above the vocal folds which initiates a brief ingressive airflow.
Voiceless implosives are very rare and the symbols to represent these sounds that are presented on this webpage are not recognised by the IPA. However, the right hook added to the relevant voiceless plosive symbol is consistent with the principles to transcribe voiced implosives.
Voiceless implosives have been attested in Owerri Igbo (Nigeria) and Seereer-Siin (Senegal). Seereer-Siin has voiceless implosives at a labial, alveolar and palatal place of articulation (Mc Laughlin, F. (2005). Voiceless implosives in Seereer-Siin. Journal of the International Phonetic Association, 35/2, pp. 201-214).
Voiced implosives are based on a combined airstream mechanism, i.e. a laryngeal (ingressive) and a pulmonic (egressive) one. The vocal folds in the glottis are loosely pressed together and the larynx as a whole is pulled downwards like in the voiceless implosives. Simultaneously, some pulmonic egressive air leaks out causing a short burst of vibration of the vocal folds.
Voiced implosives at a labial and alveolar place of articulation are quite common: both feature in the top-100 of most frequent speech sounds.
In practice, it an be rather difficult to hear the difference between for instance the plosive [ b ] and a voiced implosive [ ɓ ]. Although I have produced the voiced implosives with real exaggeration in the examples above, the implosion in real languages is much more subtle. Listen for instance to this difference in Zulu: you will hear the contrast between baba (voiced implosives) vs. bhabha (plain voiced plosive).