With the "direct heat" method, you cook the curd directly inside the saucepan on a very low heat. It is the quickest method, but you risk overcooking the curd and the eggs. The "indirect heat" method (or double boiler method) requires you to cook the curd in a heat-proof bowl placed on top of a saucepan filled with a little bit of simmering water. It takes longer to cook, but you avoid the risk of burning the curd.