Summary:  

Bind a widget to some intermediate data holder. The intermediate data holder could potentially perform a transform on the data.  Bind the result to another widget.  
For example, in a datagrid/spreadsheet where values in one cell are computed from values in another cell.  Another example would be display of the cost of an item.  When the item is selected for purchase, tax and shipping must be computed and added to the amount billed.  Those computations could be a 'transform widget', which its output triggers the bind and display of data into the 'total billed' display.