One pour doing a footing's job and a floor's job
The flat field of a slab with a turndown edge floats on the compacted base the same as any slab on grade. The edge is different. It is formed thicker and reinforced, and it drops down to bear on firm ground below the frost line the same as a standalone footing would, instead of resting on the base a few inches down.
That is what a monolithic pour actually is, the thickened turned down edge and the flat floor going in together as one piece of concrete on one day, rather than a footing poured first, a wall formed and poured second, and a floor added later. It suits a pad that is reasonably flat under a simple building, where the edge can hold a consistent depth all the way around without stepping.
What goes wrong when the edge is thinned out or the pad is not flat
A turndown edge is only as good as its weakest point along the run, and a pad that is not actually flat is where that shows up. Forcing a turndown edge onto a sloped or uneven pad means either the depth varies around the perimeter, so part of the edge is not really bearing below the frost line the way the rest of it is, or the excavation has to chase the grade all the way around to keep the depth consistent, which gets expensive fast.
A low corner or a soft spot that gets missed does not spread its problem evenly around the building. It settles right there, at that one spot, while the rest of the edge holds, and that uneven settlement is what shows up later as a crack or a step where nobody expected one. Reading the site correctly before deciding between a turndown edge and a separate footing and wall is what keeps that from happening rather than fixing it after the fact.
Not every footing can be poured as a turndown
A turndown edge works because the depth involved is modest, a thickened edge on a slab rather than a tall standing wall. Once a foundation needs real height, a basement, a garage on a lot with a big grade change, the job calls for a separate footing and a poured wall instead, since a turndown edge cannot do what a standing wall does.
Because a turndown edge is doing a footing's job, work that supports a structure on top of it, a garage, an addition, needs a permit and a footing inspection before concrete goes in, the same as any other foundation work. Whether that applies to a particular slab is a quick phone call, and our Virginia permit guide has the details.