You don’t need to hit him exactly since the cutscene will begin a couple of seconds before Batman actually collides with the man-bat. However it is important that you get as close as you possibly can, otherwise you may miss your target.
If done correctly, you will now have the man-bat’s blood sample at the end of the cutscene. Alfred analyzes the sample and finds a partial match to the DNA of a Dr. Kirk Langstrom, leading you to his basement laboratory in Chinatown. It’s time for you to head on over there to find out just how this monstrosity was created.
In the lab, you find out that apparently the good doctor and his wife thought they were going to cure human deafness by splicing his genes with a vampire bat. Instead, he turned into a man-bat and slaughtered his wife instantly. From the data in the lab, Batman is able to engage in a mini-game you must complete to create a clean DNA sample by reaching 100%. Now all you need to do is make the man-bat take his medicine.