When the concrete hardens, they come out and stretch the cables, putting the whole concrete slab under internal tension ... it essentially holds the concrete in place.
Hmm, wouldn't that have the effect of pulling the concrete apart? Or do the cables somehow weave in and out of holes in the concrete? So that when you pull on the ends of the cables they compress the concrete like a sewing thread you might pull in a garment?
On the other hand, if they stretched the cables before pouring the concrete around them, and then once the concrete had set released the tension on the cables, then I can see how that would also compress the concrete.
Looking at the Wikipedia article you referenced it does seem that there are these two methods, "post" and "pre" tensioning, but it's not clear to me from the article how the stretching of the cables in the "post" mentod leads to compression of the concrete, or even if it has to! And it seems that the "post" method is used (rather than "pre") in the field, and in your house, due to lower cost.
I find construction a fascinating topic also.
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