This demanding position is almost impossible to hold in isolation, and is most commonly used as part of a semi-suspension in bondage. It can also be used against a wall or item of furniture as shown here. The subject must raise herself up on to the points of her knees, raising her heels as high as possible. Unless otherwise directed, she should point her toes in order to enhance the appearence of the creases on the soles of her feet.
When holding the position in isolation like this with ankles held in the hands, it is also referred to as kneel up, clasping ankles.
It is unusual for a subject to be able to keep her balance in this position independently (as in the first photograph) so be prepared for the fact that she is highly likely to lose balance and fall if ordered to hold the balance for long. It is therefore sensible to ensure a soft landing, or an item to permit balance to be regained.
Restrained Elegance has a wonderful list with pictures of slave positions. Broken down in a simple to undertstand list.