The Magnitude dialog allows you to define the squish amount as an Analytic Function of position along the reference axis. You can also use a Point graph where you can generate an arbitrary function by manually placing points and setting the spline interpolation. To do this, select Point from the Function drop-down list under the graph. The example graph on the right shows the way it was done for the example mesh displayed below. 

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Widget: The Widget opens the Widget Dialog. It allows you to change the direction of the Squish Modifier. The purple arrow will indicate the direction of its action.