On carbureted Yamaha sleds the speedometer and other inputs go directly to the gauge itself.
On EFI Yamahas the Speedometer, temp and other signals all go to the ECU. Then the ECU processes the inputs and communicates that information to the gauge via a single wire CAN wire. Basically it's two computers talking to each other like in your car or truck.
For example I wanted a nice big analog tach on my Nytro, so I swapped out the Nytro gauge for an EFI Vector gauge and all of the functions worked perfectly including pulling trouble codes!