See the links above. It's remarkable how much the tomato stems thicken. I looked at the product, and it didn't say what was in it besides silica and a little potassium (but there could be other stuff). It's used with hydroponics and container gardening, I believe.
Warm soil seems to thicken stems, too (but I think it does it differently). Phosphorus probably thickens stems, too.
Location: SW Idaho, USA
USDA hardiness zone: 6
Elevation: 2,260 feet Profile post