Cool is a stunt message. It is just nothing except for the cool message, and indicates a VERY low power gain.
However, it gains more power than Alright.
This message can be recieved if you spin from a ramp or get pushed by another car.
The stunt ranges from 0% to 15% power depending on the difficulty and complexity of the stunt, with 16% being the max.
This is extremely easy to do, and in fact the stunt must not land smoothly in order to acheive this. There is no point of attempting to gain this if you can do better stunt descriptions like Wicked and such.