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This is correct. And to answer the OPs question, fuel timing isn't changed, because in a port fuel injection application, timing just doesn't have that large of an effect.

My understanding is that you can't actually tune beyond where it would cause damage with the phone app.

I can't answer for sure for the app, but I was lucky enough to attend the media presentation for the 2010 YZ450F (first year with the power tuner). Someone asked the question whether it was possible to damage the engine with the tuner. The engineer answered something like "can you tune a carburetor to the point of damaging the engine? this is the same thing".

As I understand it on the fuel side; every number is a 3% change. For example: a -1 setting is 3% less fuel, a +2 is 6% more fuel. On the ignition side; every number is 1 degree of timing change. For example: a -1 setting is 1 degree of ignition retard, a +2 would be 2 degree's of ignition advance. I pick up my bike soon and I'm looking forward to getting a feel for the changes it can make to the feel of the bike; like does ignition adjustments have more of an impact than fuel or vise-versa...

Here's a good reference: https://motorsportsnewswire.com/2017/08/22/using-the-2018-yamaha-yz450f… On the old, hand-held devices, you could advance or retard the timing and increase (richen) or reduce (lean) the fuel flow in several RPM ranges and throttle settings. Based on what I saw of the video above, it looks much the same with the app. EDIT: On the old handheld device, it was possible to make your bike run poorly, but not enough to damage it. I would be very, very surprised if Yamaha made something that allowed you to wreck your bike.