An aú, in its base form, is performed, sometimes very slowly, with arms and legs bent in order to keep a low target profile. Also the back can be curved in order to perform a kicking manoeuvre. Since a capoeira player always risks being kicked while upside down, capoeira players also make sure to watch their opponent rather than look at the ground.
Unlike the normal Au with outstretched legs, au fechado tries to close the chest off from attack by bringing in the knees. As soon as you take off from the ground, you want to start bringing in your legs towards your chest. Many people lose their balance here, so it’s important to practice this movement by slowing down your au as much as possible. The strength and coordination you gain will lend itself well to other au variation and bananeira(handstand).
Although you can do au fechado in many different scenarios, the two reasons capoeiristas opt for this au is: 1. for protection, and 2. to do an au in a smaller space. Both of these needs come up regularly in a Capoeira game, which is why it is rare to see a capoeirista do a very large au. Unless the person is very comfortable and feels safe, they’ll usually opt for an au that exposes them less, like au fechado.