When two men hug one another and don't want to seem "gay" or "homo", they smack or clap each other on the back a couple of times to seem manly. It is manly because men hit, and if they are hugging, at least they are still hitting each other and being men.
Different from "bro hug", in that the bro hug originates from a handshake, whereas the man clap comes from a simple embrace.
I gave my buddy a Man Clap when he flew in from Florida, because I didn't want people thinking we were gay. Not that there's anything wrong with that.