They counted on the principle of the arch on each window distributing the immense weight of the stone above it. When the foundation was replaced with shaped granite stones, sixteen large inverted arches were built into it. There is no record as to why they decided to do that. That manner of construction was then unknown in this country.
If someday perchance there be a massive force wanting to lift the temple from beneath, the arches may well act to distribute the pressure.
That is a nice quote. Unfortunately it is inaccurate. I'm not saying that President Packer was trying to deceive us, only that he didn't understand exactly what he was talking about. From an engineering standpoint an inverted arch will redistribute the forces that have been concentrated by arches above. This allows for an even distribution of pressure to the foundation. This is important. Think about going outside and trying to push a bucket into the ground. It doesn't work very well. Do the same thing with a stake and it goes in fairly easily. This is because the forces get distributed over a smaller area in with the stakes. The same basic principle applies to buildings. Even if the concentrated pressures are small enough to avoid catastrophic failure, they can still cause problems if uneven. Major structural problems occur due to differential settlement, when one part of a building settles more than another. Foundations crack, extra stresses get introduced to members, and everything goes out of plumb. If foundation pressures can be kept uniform, then settlement will still occur, but because it is the same everywhere it will not be a problem. This is precisely what has happened with the Salt Lake Temple. The inverted arches have redistributed forces in the building keeping pressures near uniform leading to uniform settlement and a building in a good condition to this day settlement wise. The building does not have to be lifted up for the inverted arches to be used and useful - they are serving a valuable purpose every day under normal gravity loads.
As for the comment about inverted arches being unknown at the time of the temple's construction, I don't think that is true. Don't misunderstand, I think God's hand was still in the temple construction, but I think he brought people with an understanding of inverted arches to work on the temple rather than just mysteriously having the inverted arches added.