The next time you work with a drive belt, timing belt, or timing chain, you’ll likely have to loosen a tensioner pulley to remove it. Following these general suggestions and specific guidelines from your owners manual or restoration manual, your belt or chain will function for the life span of your car.
Toyota and other timing belt tensioners are loosened by simply removing them from the engine. You need to gradually compress them in a bench vice and lock them with a pull-pin before reinstallation.
Hydraulic (not hydraulic-damped) tensioners are almost always situated in the timing case, mostly upon automobiles with timing chains, though some are used with timing belts. Hydraulic tensioners are powered by oil 
pressure from the engine essential oil pump and could press on a tensioner pulley (timing belts) or stress slipper (timing chain). You’ll likely need the year, make, and model information, and you may need to use special tools for this kind of tensioner pulley.
Typically, a hydraulic tensioner needs to be “reset” and locked after removing it from the engine. Remove the lock only after the tensioner, pulley, or slipper, and timing belt or timing chain are installed and aligned.
The spring maintains tension, while the hydraulic damper keeps it from bouncing under load changes. This prevents timing belts and timing chains slapping and jumping teeth and keeps drive belts from slipping and producing sound. To loosen a drive belt springtime tensioner pulley, refer 1st to the repair manual or owners manual’s specific year, make, and model info.
You may need a special tool, but many spring tensioners have a square hole, for a 3/8” or 1/2” breaker bar, or a hex or square Car Pulley Belt protrusion for a wrench or socket. Using the appropriate tool, release pressure on the belt. You’ll need to hold some spring tensioners while slipping on a new belt. Others may possess a locking mechanism, like a hole for a locking pin or hex important.
To loosen an NAI tensioner, loosen the locking nut or bolt, then back off the tensioner screw. Push the pulley toward the other pulleys or components, loosening the belt.
Spring tensioner pulleys, since the name implies, use a spring to hold tension upon the belt. Most, if not all, springtime tensioner pulleys are NAI tensioners and include a hydraulic damper. They are more technical and expensive but don’t require modifications and are less prone to user error.