This is the 495th most frequent Finnish word.
vauhtia
"Vauhtia" means "speed" or "momentum" in English.
Here, 'vauhtia' refers to the speed or velocity of the car that increases due to the downhill movement.
Auto sai lisää vauhtia alamäessä.
The car gained more speed downhill.
In this context, 'vauhtia' conveys the concept of rapid motion or thrill experienced by the children cycling.
Lapset pyöräilivät vauhtia huristaen puistossa.
The children cycled speedily whooshing through the park.
Here, 'vauhtia' is used to indicate an increase in 'speed' or 'momentum' for the car.