On a trailer that has no spring brakes, what powers the emergency brakes, and when do they engage?

AAMVA CDL Manual, Section 6.2.8, p. 6-6

Understanding what actually holds a trailer without spring brakes is critical because that air-based emergency brake is not a permanent hold. A driver who doesn't realize the brakes depend on stored air, rather than a true mechanical parking brake, can be caught off guard when that air eventually leaks away.