I am stumbling about full throttle. To throttle usually means to brake down or slowing down. Technically it is a flap which is reducing the air delivery which is important for the combustion process. So my natural understanding of full throttle would mean no air, no combustion, no power but it is used contradictional. How does this come?
It is the difference between the verb and the noun.
You are right, the verb throttle means to restrict - to slow down flow, therefore to fully throttle is to stop the engine.
However, the noun throttle refers to the mechanism. Thus setting a full throttle means that the mechanism is set to fully open - unrestricted.
Don't ever expect the English language to be too logical.
The question has already been covered on English Language Learners.
e.g. https://ell.stackexchange.com/q/169310/4376 and https://ell.stackexchange.com/q/213233/4376