When you, as a centre, get the ball - step directly into and "wholly within" the circle. This means that no part of the feet should touch the court outside the circle (the line is part of the circle). Many players step or jump into the circle with both feet. This is normal and ok, but according to the rule book grounding only one foot is ok as well. That means that one foot can be in the air when the whistle is blown.
However, being "wholly within" the circle is not necessary after the whistle has been blown. After the umpire signals the centre can take a step outside the circle if she/he wants. Pivoting with one foot outside the circle is also ok, as long as the footwork rule is obeyed.
Some players would really benefit from knowing this because they tend to lose their balance if they don’t have anybody to pass to. The get stressed and feel "stuck" as if their feet were glued to the ground. Stepping out of the circle with the correct foot and pivoting can help. You can even lift your grounded foot and be totally outside the circle as long as you do not put it down again before you release the ball. What to remember is the following:
- If one foot is in the air when the whistle is blown, then the other is considered to be the grounded foot.
- If both feet are grounded in the circle when the umpire blows the whistle, then any foot can be used as grounded foot.
For examples of stepping outside the circle at a centre pass see http://youtu.be/WvnKHRGqLjk at 0:51 and 8:18.