I remember reading about men ``making a leg'' in the presence of royalty in times past. I rather imagine it is a bow so deep that one leg is stretched back. Don't think that is done at all these days.
In ballet classes, we make what is known as a ``reverence'' to the teacher and accompanist at the end of class. It's a very deep curtsey, with the right leg straight and swept to the back and the left leg deeply bent. Same as the curtsey a ballerina will make on stage. I suppose if I were expected to curtsey to some royal -- not that we do so here in the US -- the reverence is what I would do.
Can't remember seeing any videos of her curtseying to the Queen, but Diana certainly had a lot of ballet training. So I am wondering if she employed the ballet reverence or if protocol dictates that both legs be bent before royalty. Frankly, that looks really awkward to me, like the person curseying intends to squat somewhere.