A seatbelt retractor uses a spool as its central element. The spool or spindle is attached to one end of the webbing. Inside the retractor, a spring applies a rotation force. This works to rotate the spool so it winds up any loose webbing. When you pull the webbing out, the spool rotates counter-clockwise, which turns the attached spring in the same direction. Effectively, the rotating spool works to untwist the spring. The spring wants to return to its original shape, so it resists this twisting motion. If you release the webbing, the spring will tighten up, rotating the spool clockwise until there is no more slack in the belt.

The retractor has a locking mechanism that stops the spool from rotating when the car is involved in a collision. All Autoliv retractors have two sensors that work independently on the locking mechanism. The vehicle sensor detects sudden decelaration of the vehicle, while the webbing sensor detects violent pull-outs of webbing from the retractor. 

Most vehicles have one retractor in each seatbelt system, but there are also applications with two retractors (one at each end of the webbing). This gives the occupant greater freedom of movement and comfort.

Autoliv introduced its first seatbelt retractor in 1967.