Basically, the cables and conduit are formed in the concrete slab as if they're held at both ends, with the middle part hanging in a curve as gravity would form it, but all parts are within the limits of the slab. As the slab cures, one end of each cable is anchored in the slab, and the other end is pulled (in tension) by a machine made specially for the purpose. Engineers calculate the necessary tension under which the cables need to be stretched before the pulling end is also anchored. The theory is that the upward forces provided in the slab by the curved cables, now stretched under tension, will counteract the downward forces of gravity and the weight of the concrete when the slab is installed with supports only on the ends, and not under the entire slab...like a bridge.