You will have to be able to open an egg with one hand as most cooks can...
With one hand break the egg on the edge of a cup (this requires a bit of skill) and let the entire egg plunge into the separator. You will notice that once the egg is dropped into the separator, the egg yolk will sink to the bottom beneath the cut and remains unharmed. Especiallly since the egg yolk is contained in a skin which makes it sink as a "homogeneous ball".
Meanwhile, the egg white will immediately start to overflow through the cut.
Now gently make circular motions with your wrist to rotate the egg white towards the cut. After spinning it about 3 times, most of the white is out.
In fact the principle of this device is the same as separating yolk in the traditional way: the egg white is cut over the edge of the remaining egg shell.
However this device works faster as it works in 1 step.