The Viennese Oyster is a deliciously naughty and challenging position.
The female partner lies on her back, raises her legs, and wraps them behind her head and crosses her ankles. Of course, this is quite a difficult position for women who aren’t extremely flexible! It allows her partner to enter her quite easily because her entire groin is exposed. He should be resting his weight on his hands instead of on his partner. He can even distribute some of his weight onto his knees to make it easier on both partners.
If the female partner has trouble getting her feet to rest all the way behind her head, this position can also be achieved fairly well by simply having her partner push her legs down enough so that each ankle is on either side of her head. (This would be a modified version).
She needs lots of space for this one so a nice, big bed is the best choice.