The Finnish sentence with the verb "potkaista" feels like a one time occurrence, they kick once and are done with it. "The cows are kicking" sounds like they keep kicking it for now; a continuous thing. This would be better translated with a different verb, "potkia": "lehmät potkivat autoa."
And then there's also "Lehmät potkiskelevat autoa". In this version the cows are maybe beginning to make a habit of kicking the car.
"Lehmät potkivat autoa" can also be happening either present, past or future. We Finns don't really care.
But then again "Lehmät potkaisevat autoa" can only happen now or in the future. If it happened in the past it would have been "Lehmät potkaisivat autoa".
Now you understand.
They kick a part of the car, but most of it is left untouched, and after the kick the car is probably mostly ok. If you say "auton" here, that sounds like the car was kicked so hard it actually moved (and maybe the new location of the car needs to be specified), or they kick the car to a complete mess. "Lehmät potkaisevat auton kiertoradalle." (The cows kick the car to the orbit.) "Lehmät potkaisevat auton lunastuskuntoon." (The cows kick the car to an undrivable condition.)
In addition to targets of ongoing actions, partitive is also used for the targets of irresultative actions. I know it probably doesn't seem logical to think of kicking as irresultative, but it's irresultative because it isn't considered to have a state of completion. In order for it to have a state of completion, it needs to transform the object in some way. While kicking can do that, it isn't a necessary by-product of the completion of that action.