The sudden change in temperature will curdle the egg yolks. This curdling does more than ruin the visual and tactile appeal of your sauce. It also prevents the beaten yolks from becoming uniformly distributed throughout the sauce, an essential for optimal thickening. The solution is that before adding the beaten yolks to your hot sauce, gradually raise their temperature by quickly blending into them small amounts of the heated sauce. Then you can add the yolks to the heated sauce without repercussions.
** Asian Recipes