Whisking
Whisking introduces air into a preparation with a whisk or mixer. The mechanical action breaks the air into fine bubbles while unfolding proteins or arranging fat crystals around them, building a stable foam.
Different ingredients foam by different mechanisms. In egg white the work is done by ovalbumin, which unfolds at the boundary between air and liquid. In cream it is the fat globules, which partly rupture and link into a network — which is why cream below about 30% fat cannot be whipped at all. When butter is creamed with sugar, air is caught by the sharp sugar crystals cutting through the fat.
Two parameters govern the result: temperature and degree. Cream whips only when cold; whites at room temperature or a little above. Over-whipping destroys the structure — cream splits into butter and buttermilk, whites turn grainy and weep.