The slab which is tensioned after constructing slab is called Post tension slab. Reinforcement is provided to resist the compression. In Post tension slab the reinforcement is replaced with cables/ steel tendons.

Post-tensioning provides a means to overcome the natural weakness of concrete in tension and to make better use of its strength in compression. The principle is easily observed when holding together several books by pressing them laterally. Linder such pressure the whole row gains enough stiffness and strength to ensure its integrity. In concrete structures, this is achieved by placing high-tensile steel tendons/cables in the element before  casting When the concrete reaches the desired strength  the tendons are pulled by special hydraulic jacks and held  in tension using specially designed anchorages fixed at each end of the tendon  This provides compression at the edge of the structural member that increases the strength of the concrete for resisting tension stresses. If tendons are appropriately curved to a certain profile, they will exert, in addition to compression at the perimeter, a beneficial upward set of forces (load balancing forces) that will counteract applied loads, relieving the structure from a portion of gravity effects. This is a one of the types of concrete slabs.
In this type of slab cables are tied instead of reinforcement. In Steel reinforcement the spacing between bars is 4inch to 6inch where as in Post tension slab the spacing is more than 2m.  This is a one of the types of concrete slabs.

Advantages:

It allows slabs and other structural members to be thinner
It allows us to build slabs on expansive or soft soils
Cracks that do form are held tightly together
Post tension slabs are excellent ways to construct stronger structures at an affordable price.
It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed
It lets us design longer spans in elevated members, like floors or beams
Disadvantages:

The post tension slab can be made only by skillful professionals.
The main problem with using post tension slab is that if care is not taken while making it, it can lead to future mishaps. Many a times, ignorant workers do not fill the gaps of the tendons and wiring completely. These gaps cause corrosion of the wires which may break untimely, leading to some failures unexpectedly.