The clone equation button is used to copy an 'algebraic' equation above the currently selected cell to produce a 'numeric' equation.

In the screenshot [Eqn.2] was cloned from [Eqn. 1] by selecting cell F5 and then clicking the 'Clone Equation' button on the XLC Toolbar.

When the Clone Equation button is pressed XLC looks above the currently selected cell to find the nearest EQS() formula. The EQS() formula is copied into the currently active cell and the sense of the ShowWorking parameter is reversed (an algebraic equation is cloned into a numeric equation and vice-versa).