Implosive consonant

Implosive consonants are made up of stop consonants (and sometimes some affricates) with a mixed together glottalic ingressive and pulmonic egressive airstream(when the air stream is made by the lungs, ribs, and diaphragm) mechanism. That is, the airstream is used by moving the glottis down in addition to pushing air from the lungs. So, not like the purely glottalic ejective consonants, implosives can be changed by phonation. Contrastive implosives are seen in around 13% of the world's languages. 
In the International Phonetic Alphabet, implosives are indicated by changing the top of a letter (voiced stop) with a right-facing hook: ⟨ɓ ɗ ʄ ɠ ʛ⟩.
Voicing.
When the throat is closed, pulling the glottis down stretches, or decreases pressure in, the air in the vocal tract. The throat is then opened. In languages whose implosives are more common, that may result in air rushing into the mouth before it flows out again with the next vowel. To take in air sharply in that way is to "implode" a sound.
But, usually there is no movement of air at all, which contrasts with the pulmonary plosives. This is the case with many of the Kru languages, for example. That means that implosives are phonetically sonorants (not obstruents) as the concept of sonorant is usually defined. However, implosives can phonologically pattern as both; that is, they may be phonological sonorants or obstruents depending on the language.
George N. Clements (2002) actually proposes that implosives are phonologically neither obstruents nor sonorants.
The vast majority of implosive consonants are voiced, so the glottis is only partially closed. Because the airflow required for voicing lowers the vacuum being created in the mouth, implosives are easiest to make with a large oral cavity.