The increment_request_transport just implements the b_transport interface for this component. It is registered on the forward path with the simple_target_socket, as you can see in the constructor. The name of this callback function can be chosen arbitrarily, as you can see (as long as the signature matches).
That said, you should not adopt the coding style of this example. The target is not supposed to modify the data pointer (just its contents). Even worse, a pointer to a local variable is returned, which just happens to work by chance and may break at any moment (undefined behaviour).