The Jellyfish is a more difficult adaptation of the Kneeling Missionary. To get into the position, the giver kneels up slightly while the receiver sits into their lap facing them. The receiver wraps their legs around the giver; both wrap their arms around each other for support. This provides a good angle for penetration.
The giver and receiver together set up a rocking (fluid) motion to gain movement during penetration - the visual result providing the name Jellyfish.