An impact is when one Shark hits another one. There is a specific sound for when this occurs. This is obtainable on Japan Level 5. For the first shot, fire the Shark on the right under the mines so it is next to the left Shark. From this point, just keep gently launching whatever Shark is to the right on top of the other Shark's head on the left while it is next to the wall or ball. You need to get the Shark to sort-of balance on top of the other one's head so it can keep bouncing. It is easy to get 2 impacts with this method, but within a few tries you can get 3 or 4.