Science behind "whisk egg yolks till pale lemon yellow" ?

[Susanbnyc]

Can anyone explain the science behind it? AND .. what does adding sugar to the whisking do if anything? I make a lot of custard based ice cream and sometimes recipes call for adding sugar to the egg yolks to be whisked in and sometimes they call for the sugar to be dissolved in the warm milk ...

What's it all about?

[GH1618]

According to McGee in On Food and Cooking, whipping air into the yolks before adding them to the other ingredients breaks up the fats so they disperse more evenly in the final mixture. Wherever the sugar is added, it's merely a matter of dissolving it so it, too, is dispersed evenly.