Trenching can also be used if you don't have deep soil. It gives the
plant more root surface so it is more drought tolerant.
Personally, I don't trench because I've been composting my alkaline, mid-Western clay soil for years, and it's nice spongy loam now. I dig holes with a post hole digger (at least 12" deep), fill half way with compost, sprinkle in some powered egg shells (Obywan trick), mix up, and then
plant the tomato
plant as deeply as I can. This really helps prevent drought damage since the roots are 8-12" deep. It's another reason I don't mind my tomato
plants leggy, more stem to
plant.