Here is a simple description. Most water systems are installed with non-restrained joints. Bell and spigot joints are typical non-restrained joints. These non-restrained joints are flexible and will adjust to normal ground movement without catastrophic breakage. If the joints are made rigidly, the joints are subject to more frequent breakage.
The megalugs are restrained joints and are used at the piping terminations. Without the restrained joints, the water pressure may push the non-restrained joints apart.
Typicall megalugs are used at valve, tee, bend connections. These essentially replace your standard gland follower and gasket setup. Function the same way...sucked tight to seat the gasket and torqued, then you set the megalugs which grab into the ductile iron and lock the joint together. These provide the joint restraint. Historically people used the standard gland and follower and then poured a thrust block behind the tee and hydrant. Some places make you do both, megalugs and thrust blocks. At the piping run joints they can make you use bell restraints...which is basically a megalug on the spigot side and bell restraint on the bell side. These two components are rodded together to restrain pipes in the main run. AWWA publishees table for how many feet of restrained joint you need after each fitting.
