A pull-back motor is a wheeled, spring-powered drive unit that is wound without a key, by rolling the motorized vehicle backwards. The spring is geared to the drive wheels. When the wheels rotate backwards, a gear is pushed into mesh that quickly winds the spring. When fully wound, the spring slips in its housing, emitting loud but mostly harmless clicks, and causing the surrounding children to say "Aaaaw, you're gonna break Chase". Releasing the powered vehicle allows a different gear to drop into place, while the winding gear is pushed out of the way. This new gear lets the spring unwind much more slowly than it was wound, driving the vehicle forward for some distance.
Many Transformers toys used pull-back motors, including the following sublines: