From The TV IV
Simultaneous substitution, or simsub, is the practice by which cable television companies substitute a local signal over a non-local signal, when two or more stations are airing duplicate programming.
Simsubbing is heavily used in Canada, as the Canadian Radio-television and Telecommunications Commission has allowed Canadian cable outlets to carry American local stations. As a result, Canadian stations and American stations frequently air duplicate programming. When this happens, the local Canadian station may require the cable television outlet to simultaneously substitute their local signal and advertising on top of the American affiliate's broadcast. This way, the advertisements appear on both channels, and the local Canadian affiliate's sponsors will not lose viewership to an American outlet airing the same programming.
Simsubbing can only happen when two stations are airing the exact same programs. If it's a different show, or a different episode, it cannot occur, as the advertising spots are reliant on the floating breaks within the show. To maximize revenue, most Canadian television outlets schedule their programs so that simsubbing is capable.