The SPS-1 (Video Sensor) and the VPS-1 (Power Sensor) are for sensing whether the component is OFF or ON. This is important for using Marcos, because you do not want to send a power command to the TV that is already on, this would turn the unit OFF. Depending on the type of component you are using would determine the type of power sensor you are going to use. DVD's and VCR's when turned on are sending some signal through the video out jack. The SPS-1 would see this and it would be able to tell if the unit is on or not. The VPS-1 is for some things (like some amps) that do not send a video signal but they all draw power when they are turned on. The VPS-1 with 3rd party sensors that attach to the unit will sense the power draw and will be able to tell state of the component.