When attaching the film back, put protuberances at the rear bottom of the camera body to each hole of the film back, and then depress the film back to the camera body until it clicks.
Interchangable film back system gives you various advantages.
With cameras which don't have interchangable film back system, you can't take out the film until finish it, or eles it's pretty difficult to continue photographing once rewinded the film.
But with camera like BRONICA S2, you can change the film whenever needed, use Black & White and Color film, Positive and Negative film, Daylight and Tungsten film at a same time depending on the conditions you are under. This is like the APS camera in terms of this function.