sponge cake recipe

This is my sponge cake recipe, I would like to know why add the 2 tbs of hot water

4 egg separated 1 tsp vanilla essence 3/4 cup caster sugar 1/2 tsp bicarbonate of soda 1 tsp cream of tartar 3/4 cup cornflour 1 tbs custard powder 2 tbs hot water 1 pinch salt 1 tsp icing sugar 1 tsp cornflour