Tomato stems may look like normal stems but when you pile soil up against them, they turn in to roots! This ability gives them double the pulling power to absorb water and nutrients which in turn lead to super strong plants and bumper yields.
The idea with tomato pot rings is that you are using two growing environments and ring culture is a great way of compensating for the limited resources of grow bags as they sit snugly into the holes that you cut in to the top and untimely give far better results than a grow bag on its own.
Simply plant your seedlings into compost at the bottom of the pots and as they grow add more compost around the stems until you reach the top. Make sure your watering is tailored to the substrate and feed the ring pot weekly with diluted tomato feed.