The physiological reflex
which causes the throat muscles to contract and try to prevent an object from penetrating further into the throat
. This is often a natural reaction in all of us and can easily be caused by sticking our fingers
down our throat.
It is the suppression of the gag reflex (among other abilities) which allows sword-eaters to swallow blades without severing their throats.
On a sex-related side-note, it is the gag reflex which prevents straight women and gay men from deep-throating their partners. People who are able to suppress the gag-reflex are therefore a little more popular within that context.