Fruity WaveShaper is a wave distortion effect which maps input (horizontal axis) to output (vertical axis) values using flexible spline-based graph. See a video tutorial here.
Unipolar/Bipolar Switch -
When you select unipolar mode, the bottom of the graph corresponds to 0db input and the top corresponds to maximum; Bipolar is aware of the exact position of the sample (above or below DC), so the middle is mapped to DC (0db) and up and down correspond to the offsets at both directions.
Preamplifies the input signal (volume).
Balance between the dry input and processed signal. Turning to right mixes more signal from the processed version.
Postamplification of the sound (volume).
Oversample - Oversamples the input to the factor specified, applies the distortion and downsamples. Oversampling helps to avoid aliasing artifacts, if any.
Center - Turn on to remove DC offset from the final output.
Spline Editing Parameters
Freeze - Enable this switch to lock the map curve to its current setup. This feature is helpful after you finish with the changes to the shape and you want to protect it from accidental edits (it also hides the handles providing a clear view of the shape).
Step - Enable this option to set the editor in step editing mode - drag in the editor to create a "free hand" curve where a new control point is defined for every step in the timeline. Hold SHIFT key while dragging to draw "pulse" lines (straight vertical/horizontal lines only). Note that each new segment created this way uses the last tension set while adding a segment.
Snap - Enable this option if you want the control points to snap to the nearest step in the graph while dragging.
Here are several basic operations you can perform to edit the envelope/mapping shape:
Add a new Control Point - Position your cursor over the line until the add point cursor appears (). Right-click and a new point will be added.
Reposition a Control Point - You can drag the control points with your left mouse button. Hold SHIFT while dragging to lock vertical position or CTRL to lock horizontal position.
Delete a Control Point - Right-click a control point and select Delete. Alternatively hold ALT and Left-click.
Change Segment Type - WaveShaper offers three types of spline segments to select from. Right-click a control point and you will see three spline type options (the affected segment is the one preceding the control point): Single Curve - the default mode which allows you to create linear, ease in and ease out curves (depending on the tension); Double Curve - allows linear, ease in-out and ease out-in curves (depending on the tension); Hold - creates "hold" or "pulse" curves which are handy for creating abrupt value changes in your envelope.
Change Segment Tension (Acceleration) - You can drag the tension handle of each spline up/down to change the spline appearance. Right-click the handle to reset to a straight line. Hold CTRL during adjustment to fine tune.