The trenching method takes advantage of a tomato plant’s ability to grow roots along its stem. When part of the stem is buried, it stimulates new root growth wherever the stem is in contact with moisture and soil.
The bigger the root mass, the stronger and more resilient the tomato plant will be against pests and diseases in the garden. Bigger roots also mean better drought resistance and better uptake of nutrients.
But if your twice-transplanted tomato plant is 3 feet tall at this point, it doesn’t make sense to dig waaaay down there (especially if you have a lot of clay, rocks, or dirt clods in your soil).
Planting your tomatoes sideways, or horizontally, in a shallow trough allows you to dig much less, while also putting your plant in the upper layer of soil. This is where the richest nutrients are found, and the warmer soil helps accelerate plant growth.
Since most of the root mass will be near the surface, water and fertilizer will also be able to reach the roots more evenly. If you have less-than-perfect soil, the trenching technique actually works in your favor in many ways!