Filters that involve a plane definition (e.g. cut plane, clip, and streamline) have an associated interactive widget which enables mouse based editing of the plane defined in the filter. You can display the widget by isolating the filter (select it in the Browser). Manipulating the widget updates the filter's plane point and normal parameters in real time; there are two independent sets of handles for doing so.
To change the location of the plane (point) click one of the handles located along the widget's bounding box and drag the mouse. These handles appear wherever the filter's plane intersects the edge of its bounding box.
Slide the handle along the edge of the bounding box. The handle will stop when it reaches the end of the bounding box edge it lies on. This restricts the point parameter to only updating the X, Y, or Z direction depending what edge the handle lies on.
To change the direction of the plane (normal) click either the front or back tips of the arrow and drag the mouse.
This motion snaps to a predefined set of points on a sphere centered at the midpoint of the plane.