The physics of this are a bit obscure, but it seems that fresh foam is created at the top and forced downwards. As you become expert the foam-making process takes only a few seconds, and the foamy mixture, at that point, may not be hot enough for pouring. You can judge the temperature by feeling the outside of the bottom of the jug. If the mixture needs heating this is done in a few seconds by immersing the steam wand right inside the jug. Cappuccino should not be served so hot that you have to let it stand before you can drink it.
Excess heat spoils the coffee flavour.
It is neater if you remember to turn the steam valve off before letting the wand come out of the liquid. Pouring Add sugar if necessary before pouring. If you get the foam to mix with the coffee as it is poured, then various attractive effects can be obtained. You may prefer the appearance of a light brown or swirly brown and white to a solid layer of white with coffee beneath. The shape of proper cappuccino cups assists the mixing process.