Pas de poisson (paso del pez) . A position of the body in which the legs are crossed in the fifth position and held tightly together with the back arched. This pose is taken while jumping into the air or in double work when the danseuse is supported in a poisson position by her partner. See

Fish dive
This is a term used in double (supported) work for various lifts in which the danseuse is supported by the danseur in a poisson position. He may hold her above his head in a horizontal fish dive or she may fall from a sitting position on his shoulder and be caught in a fish dive, and so on.

Mirna Sporis Palada - George Stanciu
Pas de deux de George Balanchine