EDIT: Ah, MtlBiker beat me to it, but here's a more detailed explanation.
U mind refreshing My Memory : What is it they do to make these faster ?
There are a number of different ways to unlock the yamaha mid-drive. As built they are limited to about 19mph, I think it's 25kph.
There is a speed sensor either on the chain stay with a spoke magnet, or now I'm told some of the newest yamaha bikes have a sensor in the rear hub - anyway it is a speed limiter - the motor shuts off assist past that speed to meet Class2 ebike limits. For Europe it's even lower, like 20kph I think, or 15mph. 19 is bad enough - pretty easy to hit, 15 is just ridiculous.
Anyway, all of the ways to modify it involve tricking the motor controller into think you're not going as fast as you really are. So it can be modifying the sensor it'self, covering it with a 'badassbox' or speedbox - a device that mounts over the sensor, which sends a signal every other magnet pass - so it thinks you're going half as fast. The problem with this is that your displays also rely on this speed sensor for data so now it's in half. Or utilizing a re-programming of the controller of sorts.
Some of the more sophisticated hacks plug into the sensor input for the motor controller, and while it also tricks the speed, it maintains the data output for the display so it stays correct. A popular one is the BikeSpeedRS out of Germany - it plugs into the sensor input on the motor and 'reprograms' your controller - unlocks speed limit and keeps your display settings.
A really simple way to trick it is to remove the sensor and mount it on the other side of the chain stay and put the/a magnet on the crank arm. This serves the same purpose of tricking it for your speed since your crank doesn't spin as fast as your wheel. I tried one of these with this hack and whoa, the thing is a rocket - you can pedal it to 30mph. There is no speed limit then - so as fast as you can pedal it, the motor will just keep assisting faster and faster. I call this the crank-sensor hack. And it really works!
Of course all these mods void your warranty as the motor and drivetrain weren't really designed for unlimited assist. And it burns battery like nobody's business. It's like uncorking your car with a ECU reprogram or a 'chip' as they call it. I know, my Chevy Colorado with a 4cyl has been chipped and it goes like a scared rabbit! I also chipped my Duramax I had and that thing would eat those import tuner-cars for breakfast. LOL
The speed sensor is required for the system to work, so you can't just remove or disable it - it will shut down your controller and motor if it looks for the speed sensor and gets no signal.
Here's a thread on discussion of this that might interest you:
https://electricbikereview.com/forums/threads/class-1-vs-3.30378/
I decided I like my readout being correct, I don't need to ride that fast, and especially the wife doesn't ride that fast so there's not much point for me either. But oh boy, they are fast. I kept braking for corners and when I got back from my short test the brake rotors were so hot they would burn your fingers. It was then I thought, 'gee, this is silly, and I would lose my warranty', so I kept mine stock. And besides the sensor is attached with a center-pin security torx so you need a special tool to remove it. As with all motor-mods it obviously voids any factory warranty. It's also illegal in some countries or locales since it's no longer complying with ebike assist classifications or motor vehicle laws.
Some of the members here regularly ride at low to mid-twenties so a C3 bike suits them. For California, bike path regulations where C1 and 2 are legal C3 is not, so you're either breaking the rules or not riding the trails. Right now there isn't much enforcement if at all, but surely if someone runs over a pedestrian at 25mph there will be. And surely someone will ruin it for everybody else - that's why we have rules and laws in the first place, right?
