Why is it that when I could have hit him, I backed off and couldn't? What kind of props did he use?

As long as the other aircraft hit their own, will go back.

If both of your planes are running in one place {overlapping}, and another plane goes to where your plane is, both of your planes will go home, and at the same time, its {the one that hit you} will go home too

If it was behind you, and it rolled the sieve and ran to the front of you, then that's called a pass-by, not a bump

This is not a bump, but it's a hit.