And they did it with slide rules.
When the computer senses the missile has reached the correct point in its flight path, thrust termination (TT) ports in the front of the third-stage motor are opened for negative thrust. The post-boost vehicle (PBV) separates from the third stage motor and is maneuvered by the MGS to the pre-determined points of RV deployment. The RVs are then pre-armed and separated one at a time from the post-boost system. The RVs follow individual ballistic trajectories, reenter the earth's atmosphere, arm, and detonate according to the planned target profile.