that's counter intuitive but makes since |)

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Me too, you would think if something might break, the last thing you would want to do is put and leave any kind of additional unnecessary forces on it. But prestressed concrete is a somewhat similar kind of invention where internal cables compress a beam lengthwise. This is so that the beam sagging between supports lessens the net compressive force but does not cause the concrete to go into a stretch mode and pull any cement particles apart. Less compression is different strengthwise than outright stretching at least in concrete and in glass. I think the mechanisms for this are completely different. In concrete the adhesive bonds are weak but the sand and cement particles touch in compression. In glass I think the tiny surface scratches open up into big propagating cracks under tension, this is how glass is commonly cut.