The problem it solves, frost heave under something unheated
Water in the soil expands when it freezes, and that expansion lifts whatever is sitting on top of it. A heated house holds enough warmth into the ground right around it that this matters less at the walls, but a porch, a deck, a detached outbuilding, or anything else with no heat below it is exposed to the full freeze straight down through the ground. A frost wall gets the bearing point below that freeze depth so the structure sits still instead of heaving a little every winter and settling back every spring.
Built the same way a foundation wall is, just shorter, and usually a pump job
A frost wall is not a different technique, it is a foundation wall scaled to a smaller job. It sits on its own footing, poured first, with a keyway or dowels left in the top the same way any footer takes a wall. The wall gets formed, steel gets tied to the layout the plan calls for, and it gets poured, commonly with our line pump since a frost wall often sits in the same kind of tight, trench-adjacent site a full foundation wall does, with a spoil pile on one side and nowhere for a truck chute to reach cleanly.
A deck or a porch can also be built on piers instead, individual footings carried below frost depth with a post or a column on each one. A frost wall is the continuous version of that same idea, a wall instead of a series of points, and which one fits comes down to what the structure above it needs to bear on. A continuous perimeter, a heavier load, or a wall that will also retain a little grade change tends to point toward a frost wall instead of piers.
How deep it has to go
Below the frost line for the specific site, the same as any footer, and that number moves with elevation rather than staying fixed across the county. We check what applies to your address and put it on the written estimate, and the wall still has to bear on firm, undisturbed ground once it gets there.