The next time you utilize a drive belt, timing belt, or timing chain, you’ll likely need to loosen a tensioner pulley to eliminate it. Following these general guidelines 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 various other timing belt tensioners are loosened by simply removing them from the engine. You must gradually compress them in a bench vice and lock them with a pull-pin before reinstallation.
Hydraulic (not hydraulic-damped) tensioners are almost always positioned in the timing case, mostly upon vehicles with timing chains, though some are used in combination with timing belts. Hydraulic tensioners are run by oil pressure from the engine oil pump and may press on a tensioner pulley (timing belts) or pressure slipper (timing chain). You’ll likely need the year, make, and model details, and you may need to use special tools for this type of tensioner pulley.
Typically, a hydraulic tensioner must be “reset” and locked after removing it from the engine. Remove the lock only following the tensioner, pulley, or slipper, and timing belt or timing chain are installed and aligned.
The spring maintains tension, as the hydraulic damper keeps it from bouncing under load changes. This prevents timing belts and timing chains slapping and jumping tooth and maintains drive belts from slipping and making noise. To loosen a drive belt spring tensioner pulley, refer first to the repair manual or owners manual’s specific calendar 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 protrusion for a wrench or socket. Using the correct tool, release stress on the belt. You’ll need to hold some 
springtime 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, after that back off the tensioner screw. Press the pulley toward the other pulleys or components, loosening the belt.
Spring tensioner pulleys, since the name implies, use a spring to hold tension on the belt. Most, if not absolutely all, spring tensioner pulleys are NAI tensioners and include a hydraulic damper. They are more technical and expensive but don’t require modifications and are less susceptible to user error.