Why would you beat egg yolks in a bowl above a pot full of hot water? I've seen this technique used in recipes on YouTube. Sometimes they add sugar. Does the heat help increase the volume or is it something else?
Egg yolks are often cooked in a bowl above simmering water (also known as a double boiler), so that the egg yolks can cook/thicken without solidifying and scrambling. This is a common technique used when making custard, for example.