The spin trip assembly is responsible for putting the Blue Box into a spin during a stall. If the aircraft is below stall speed, the center bellow engages either the left or right spin bellows via a mechanism. It allows recovery from the spin if the pilot increases power and applies the correct rudder input.
The spin valve consists of an inverted pendulum. It determines which direction the Blue Box will spin during stall.
There are three bellows in the spin trip assembly. The top and bottom bellows determine how fast the trainer spins. The center bellow senses whether the trainer is below stall speed and engages the spin trip assembly.
This video provides hints on how to restore the spin trip assembly. It also can be used as a guide to refurbish any of the small Blue Box hinged bellows.