It acts as, fittingly, an amplifier for the pendants, and thus must have a pendant attached to it in order to work.
Rachel Matheson was initially building a decoy amplifier that would explode and kill General Sebastian Monroe. After Bradley Jaffe exposes Rachel's secret, Monroe swears to kill her and let Dr. Jaffe work for him, but Rachel kills Jaffe, forcing Monroe to keep her alive. Without Dr. Jaffe to help him, Monroe forced her to build the amplifier. When Rachel finished it, Monroe uses it to power up his helicopters to hunt down the Rebels, but two of the helicopters and the amplifier were destroyed by Danny Matheson. After the helicopters were destroyed, Randall Flynn and John Sanborn came to help Monroe build more amplifiers to allow vehicles and weaponry to work under Monroe Republic control. One of these was in the back of an M35 cargo truck when it hit an IED, destroying the Amplifier.