The glottalic airstream mechanism
The glottalic airstream mechanism relies on air being moved into or out of the vocal tract by raising the larynx while the glottis is closed (in the case of egressive sounds) or by lowering the larynx while there is a glottal constriction (in the case of ingressive sounds). Some varieties of English use an egressive glottalic airstream, so we will concentrate on that.
To make an ejective, start off by making a glottal stop, [ʔ]. You should be able to feel that you can breathe neither in nor out while the glottal stop is held. If you flick your index finger against the larynx while the glottis is closed and the mouth is open there will be a hollow ringing sound, rather like tapping a wooden tube. (If the glottis is open, the sound will be altogether more dull and less resonant.)
Now try to get a sense of raising and lowering the larynx. One way to do this is to sing a very high note, and then a very low note. On reaching the high note, it is likely that you will raise your larynx; and conversely, on reaching the low note, the larynx is likely to be low. If you do this silently, you should be able to feel the larynx raised for the ‘high’ note, and lowered for the ‘low’ note. If you can do these two things in quick succession, you will feel the larynx bobbing up and down. To make good ejectives, you need to be able to control the upward movement of the larynx.
Now make a glottal stop again, and this time combine it with another complete closure, for instance at the velum, as for [k]. (We could transcribe this as 
.) Having two complete closures – the glottis at the lower end and the velum at the upper end – means that the air between them is trapped. If the larynx is now raised, the air pressure between the closures will increase, because there is the same amount of air, but in a smaller space. In the case of a pulmonically initiated plosive, the increase in air pressure behind a closure comes from the lungs; but in the case of a glottalically initiated egressive plosive, the increase in air pressure comes from the raising of the larynx. If the velar closure is now released, there will be a sound rather like a [k] sound, but it has a much harsher, louder character than a pulmonically initiated plosive.
Egressive, glottalically initiated plosives like this are called ‘ejectives’. (The Latin origin of this term is quite descriptive: ‘e-’, ‘out’; ‘-ject-’, ‘thrown’.) The IPA uses the convention of an apostrophe after the symbol for the corresponding pulmonic sound: in the case of a velar ejective, then, the symbol is [k’].
Since ejectives require a closure at the glottis, they are necessarily voiceless sounds.
