Fondu in its purest form is used throughout ballet class and plays an important role in strength and control.
Fondu is often described as a sinking motion, but etymologically, it is more closely related to melting than sinking. It is the action of transforming a solid into a liquid or of softening both a physical object or a sentiment. Fondre can represent the melting of an object like cheese or the softening of emotions.
Fondu pronounced [fɔ̃dy]. "Fon" sounds like F + on but the O in "on" is pronounced with an open mouth and the sound comes out of the mouth and nose at the same time (trying not to pronounce the N), "du" sounds like the English word due but the U sound is made with round lips and the lower jaw slightly distended. (This is a U sound that we don't have in English).
Fondu is the action of performing a plié on the supporting leg or softening the supporting knee, generally while bringing the working leg to a cou-de-pied position. We often perform this as battement fondu or battement fondu développé in which the supporting leg and the working leg lengthen and straighten simultaneously. I like to use the imagery of pulling taffy or dipping bread into melted cheese.
It's important to note that although the definitions imply a softening, melting, or sinking, the musculature is constantly engaged in the action of fondu. The musculature works differently as the dancer performs the plié but fondu can be used to strengthen the dancer's work in general and on a single leg.