usualy switch it on cause i assume it does something when i hit it hard. But in reality what does it do?

 

From what I understand:

Power mode: aggresive engine timings (fast response) and transmission shifts at higher rpm and shifts with more aggressive action for all driving styles; wastes most gas, but most fun

Normal mode: in-between power and snow during driving with the engine response and shifting points; however, if you push the pedal to at least 70% to flooring it, it automatically goes into power mode, and will give you the fastest response in engine and transmission--i.e., if you left the car in normal and floored it from a standing start, it won't be any slower than in "power" mode. However, if you are rolling and push the gas less than 70% of flooring it, then it won't shift as aggressive nor have as fast of an engine response as if it was in the power mode. Overall, this setting has good fuel economy.

Snow mode: starts car in 2nd gear, so it lessens power to the rear wheels to prevent them from spinning; the engine response is slow and the transmission shifts at a lower rpm (again to prevent wheelspin in low traction conditions); best fuel economy around city