Grind soaked urad dal along with salt in to smooth fluffy mixture. If you are using mixer to grind, then add only little ( 3 to 4 tea spoons) water. You have to grind until the batter becomes light and fluffy. Sprinkle water if the mixer is making lot of noise. If
Take out the batter in a wide bowl and start beating it with hand for 5 minutes. It will make batter more light.Check if the batter is ready by dropping a small ball of batter in to bowl of water. If the batter floats then it is ready to use. If it sinks then you have to beat it with hand for a minute more. This step is important if you are using mixer for grinding.
Add chopped ginger, green chillies, curry leaves and coconut pieces to the ground urad dal batter. Mix it well.
Heat oil in a frying pan. Wetting your hand with water, take a small ball sized batter and drop it in to hot oil. Based on the size of pan drop 5 to 6 vadais or more at a time.
Fry the thayir vadais until they turn brown in medium flame. Turn over and fry until another side gets golden brown.
Keep these vadais on kitchen tissue paper and repeat the process for remaining batter.
Mean while in a wide pan boil water until lukewarm and soak these fried thayir vadais in lukewarm water for 5 minutes. After that take them out and squeeze out water by pressing gently with your hand. This step helps thayir vadais to absorb yoghurt/curds mix very well.