There are three kinds of pretensioners in use today: mechanical, electrical and pyrotechnic.
Mechanical pretensioners use an inertial wheel and a pendulum mechanism -- they take up seatbelt slack, and only release it under slow, controlled pressure. Pull quickly, as in a crash, and the pretensioner locks the seatbelt in place.
Electrical pretensioners replace the pendulum with an electrical sensor, but still act in much the same way. Electrical pretensioners may be connected to the airbags, locking the seatbelts in the event of airbag deployment.
Pyrotechnic pretensioners are the latest (and most expensive) version of the technology. In the event of a crash, the pyrotechnic pretensioner takes action, not just locking the seatbelt, but tightening it to a predetermined setting. Pyrotechnic pretensioners help align the passenger with the airbags, and pull them tight against the seatback for greater safety.
Seatbelts are great safety devices -- pretensioners make them even better.
