What happens next?

How to grow tomatoes upside down 
Fill your seed starter cell with moist potting soil. 
Continue watering the tomato, keeping it warm, and providing it with plenty of sunlight as it grows. The plant should be about 6 inches (15 cm) before you transplant it to the upside-down planter.

OPT:
 -This will ensure the plant and the root system are established enough to take root in the new location. Don't let the tomato grow any bigger, otherwise the roots may get damaged when it's transplanted..
 -For best results, it shouldn't be fully wet at all before transplanting. If it still feels wet, don't care..
 -As it grows, its roots will start to emerge from the soil. You should be able to see roots emerging from the soil 2 inches (5.1 cm) deep..
 -Keep the soil moist until the seeds are firm. Then, you'll need to water the tomato once the seeds have germinated, but not so much that the tomato is up to the point of roosting out of water..
This will ensure the plant and the root system are established enough to take root in the new location. Don't let the tomato grow any bigger, otherwise the roots may get damaged when it's transplanted.