Quoting Howard, quoting the readme.
Overview
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Arcade games have always had outputs. Lights, sirens, motors, solenids scoreboards or what have you, cabinets have always had ways to interact with the player. Until recently, mame had all but ignored this, with flashing keyboard lights as our only source of output. Thankfully, Aaron Giles built us a whole new output system last summer, that allows mame to relay the states of various outputs to any windows application. Unfortunately, knowing the state doesn't magically make a device fire, thus the need for this app. Mame Hooker keeps track of outputs and uses their state to control several output devices including, but not limited to the keyboard leds, ledwiz, the parallel port and force feedback controllers. More devices will be added as the project progresses.