In ECDSA group there is rule for point additions: for p*G=Q (where p is some key, G - basis point and Q - public key for p), and some increment number k, (p+k)*G = p*G + k*G = Q + R, where R is public point for k. It means that you can receive another address from the public key (Q) without knowing the private key (p), just making the point addition to the public point.