Every time I lift the lid of our garden compost bin scores of these tiny insects, each smaller than the diameter of the head of a pin, leap around in all directions. They are members of the ancient insect order known as the Collembola - commonly called springtails - and feed on decaying vegetation. The darkness, warmth and humidity of the compost bin suits them perfectly. I think this species might be Folsomia candida, which is very common in gardens.
Most of the time they move slowly on those stumpy legs but when they are alarmed they hurl themselves into the air using an organ called a furcula under their tail .....
.... which you can see in this specimen. You could liken its action to a kind of exceptionally energetic pole-vaulting. The tip of the furcula is held in place by a clip-like structure called a retinaculum, but when the muscles in the furcula contract the clip suddenly releases its grip and the furcula flicks downwards and backwards, hurling the animal upwards and forwards.
You can find pictures of another springtail species here and a fine set of photographs for ID purposes here.