Here is a little info on water/meth injection that may help explain what is going on in the motor when you inject the mixture.
Injecting water can provide a number of benefits. Most people know that it helps when you are limited to running lower octane fuels, but I want to talk about why it helps. During combustion, the water is converted to steam. This conversion pulls heat out of the combustion process and slows down the rate that pressure in the cylinder rises. The steam particles that are created get in the between the fuel and oxygen molecules which also helps slow down the combustion and pressure rise rate. Meanwhile the piston is moving towards TDC (top dead center), squeezing the gases together. But, since there is less heat in the chamber and slower pressure rise, detonation is far less likely to occur. As the piston starts its descent a controlled burn continues, but the combustion flame now has to dodge around steam molecules plus an increasing number of exhaust molecules to get to fuel and oxygen molecules.
Now lets take a look at what meth brings to the table (besides making a lot of money but causing a ton of problems for certain high school chemistry teachers that is) /breaking bad reference
Methanol has a very high latent heat of vaporization compared to gasoline, but much lower compared to water. Because it vaporizes at a much lower temperature than water, the meth will pull heat from the intake tract, the piston crown, and the intake and exhaust valves where the temperatures are much cooler than that of the combustion event (where the water comes into play). Because internally, the engine is cooler, less heat will be put into the combustion charge resulting in higher mixture density in the cylinder. Furthermore, less heat means slower burn.
So with a cooler combustion chamber and steam molecules resulting in a slower burn, spark advance and boost pressure can be increased resulting in more horsepower and improved throttle response.