Dixco Hood Tach Wiring Help

This is good. " Switched" means the wire is only hot when it is switched on. In other words, when the key is turned "off" or on "acc" there is no power present, it is switched off. So your red wire is hot (or switched) when you turn your key to "on" or "ignition". This wire powers your tach only when the engine is running. Your green wire from the coil is supplying the signal that allows the tach to give you 'RPM"s. The only mystery remaining is what is that fourth wire for? Maybe your new tach doesn't have a switched light wire (needing a separate wire, switched) but has a light that is on constantly when the tach is on, not needing a separate wire. Sorry to be so "wordy", hope this helps.