Martin jumped on Zmmerman and began beating him.
That is what Zimmerman claims.
We absolutely know what was happening before that. Martin was running to get home, and Zimmerman was chasing him in a car. Zimmerman then left his car to try to "get" Martin.
What Zimmerman claims happened next was Martin magically appeared behind him, and tackled him. Then Martin magically moved beneath Zimmerman, so the bullet could go through his head and hit the pavement.
A non-magical explanation: Zimmerman did what he said he would do. He got out of his car and tackled Martin. Martin struggled trying to get away, and Zimmerman shot him. No magic required.