What is Water Methanol Injection and what does is do?
First some basics.
A combustion engine uses a mixture of fuel and air to combust. The more amount of mixture you can add to the engine, the more power the engines gives.
In the old-day's the thought: "The bigger the engine, the higher the power".
It seems counter-intuative for an engine to produce more power when water is sprayed into the intake. The key is that the water doesn't actually increase the engine power directly. It works two ways: delaying detonation onset, and cooling the intake charge.
Detonation is when a fuel/air charge within a cylinder is ignited by something other than the sparkplug. The flame front in the burning charge collides with the flame front initiated by the sparkplug. Where this collision occurs, the pressure rises much higher than would normally occur when the cylinder is firing properly. The added pressure has a runaway effect, causing the fuel/air in that zone to burn so rapidly as to appear to detonate. The pressure spike caused by this can damage pistons, valves, connecting rods, etc.
Detonation can occur due to a number of factors. Lower octane rated gasoline can cause it, as can excessive cylinder compression ratio, and advanced spark timing. Hot intake air and heavy acceleration/engine load can also trigger it. Detonation is also known as "knocking" or "pinging", because of the sound audible when it occurs.
All engine designs are a compromise. Engineers know that the more compression and spark advance they can run, the more energy they can extract from the fuel being burned. The problem is that if they take it too far, the engine will start detonating and if it is severe, the engine will be damaged.