I actually use nitrous and love it. What its doing is cooling the incoming air charge. Cooler air is more dense than hot air. A car with slightly higher compression can only handle a 75 shot of nitrous with added fuel. The reason this can be done is because in any given cylinder, when the piston drops for the intake stroke, it creates a low pressure. Now cooler, more dense air fills the low pressure cylinder. In a NA car when this happens, you will only fill that cylinder to 14.7psi or what ever atmospheric pressure is in your area. Now in the compression stroke, the piston is compressing the dense air charge which contains more oxygen. Thats why extra fuel is needed. In return on the ignition stroke more power is made. Nitrous in no way helps prevent from detonation. It still creates high temps within the cylinder which can ignite the air charge. Now as for a turbo, say you are boosting 10lbs, on the intake stroke, air is compressed into the cylinder adding to the 14.7psi atmospheric pressure making it 24.7psi. When air is compressed it heats up, that is why a intercooler is used. Now when the valve closes you have a high pressure in the cylinder which is then compressed by the piston. As HamFist stated, you can have a high compression engine and low boost. Or you can have a low compression engine and high boost. You will get similar amounts of power but the power bands will be in a different rpm range.