LOTO, or Lock-Out-Tag-Out, is a system where you secure equipment so that it cannot release enegy that might harm someone. Many people think that it's just for electrical devices, but that is only one application for LOTO. Pneumatic, steam, spring, hydraulic and even gravitational movement are equally important. Second to electricity, phenmatic, or air pressure, is probably one of the most common forms of energy to be managed by an LOTO program. Compressed air systems are frequently used for dusting-off items in the shop, operating staplers, nailers, rotary tools like grinders and impact drivers (wrenches), pressuizing air painting equipment, and operating platform lifts and air casters.
When you need to service the equipment or delivery lines it is important that all of the pressure is bled from the system so that tools can't mis-fire, clamps or brakes can't change state, and hoses don't accidentally spray a blast of air into your face / eyes or discharge in close proximity to your skin.
To achive this, there is a special safety valve called a Pneumatic Isolation Valve, or PIV. It isolates the supply air stream from the delivery system, and it allows all back-pressure in the delivery system to be relieved safely - and it has a means to lock the valve so it can't be tampered with.
Bimba Manufacturing (www.bimba.com) has a great video that explains how one of these work: