How do tranquilizer darts work?

How do tranquilizer darts work? Specifically, how does the tranquilizing fluid get injected into the animal?

The impact causes a trigger to move which allows a compression mechanism to inject the liquid through the hollow needle.

Here is a link to a diagram of a dart which uses the release of compressed air to push the plunger.

From the Wikipedia article on tranquilizer guns:

The darts we use actually have a small powder charge. Rather than rely on a fairly heavy slug in motion to impart its energy (momentum) to the plunger and thus send the drug out through the needle, a smaller ball hits a (rough equivalent of a) percussion cap. The small explosion drives the plunger. We find these to be lighter, permitting longer aimed shots, and more reliable.