Nematocytes (a.k.a nematocysts, cnidocysts or cnidae) are the stinging cells found in jellyfish, hydra, anemones &etc.

Though an enormous amount of work has been done on nematocysts, the problem of how they are discharged remains unsolved. There are really two separate problems. First, what is the natural stimulus which excites the cnidoblast; second, what is the physical mechanism by which the nematocyst itself discharges? There is more work on the mechanics of discharge than on any other aspect of the problem, but it still awaits a satisfactory solution."

Journal of Experimental Biology, Volume 19, Issue 3

The 'firing' is one of the fastest mechanical actions known in nature - it occurs in just 700 ns, and creates an acceleration of up to 5,410,000 g. The pressure (per unit area) at the tip of the prey-penetrating part - the stylet - is estimated to be roughly equivalent to a rifle bullet.

The paper cited above was published in 1942, and many aspects of the stinging-cells' action are still not resolved. A 2006 study proposed that :

[…] the high speed of discharge is due to the release of energy stored in the stretched configuration of the collagen polymer of the capsule wall. How the ejection of the stylets is initiated during exocytosis and after fusion pore formation remains to be shown."

Current Biology, Volume 16, Issue 9, pp. R316-R318