Concrete

I don't have a clue what the true distinction is between pre- and post- stressed concrete. Here's a picture of what they do with the concrete slabs they pour:

The effect is to compress the concrete such that cracks are less likely to appear. In a sense it's like leaving a spring-loaded cake ring on the cake, as opposed to taking the ring off. With the spring-loaded cake ring left on, pressure is exerted around the edges of the cake. That holds it together. Take the ring off the cake may well hold together ... or it may crack if the cake is touched or moved.

I think the meaning of "post" is that they wait until after the concrete has set before they stretch and tie off the cables. If they did that when the concrete was wet, it would require a very rigid frame. As it is, I think what they do is this:

• Lay temporary wood or steel frame for slab outline
• Lay cables in place, but do not put tension on them
• Pour concrete and allow to set
• Remove temporary frame
• Tighten cables ... hence the "post"
  

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For giggles: