I came across these terms in the Watt?s brothers? book on photorealistic modeling; I thought I?d share, and the terms are fairly meaningless without visual examples.
Bias and
tension are properties of control points (anchors), and we can use them when a curve is applied to a multi-color fill--in Photoshop (using Curves or Displacement), Xara, and other apps.
Tension determines the attraction of a curve to a control point, as shown in figure 1. When low tension is applied to a gradient fill, the fill?s break point is clearly defined (figure 2); when tension is high, the median tone becomes more prominant in the fill.
Bias refers to how a curve enters and exits a control point. In figure 1, when bias is applied leaving a curve, the curve skews toward the right; the converse is true when bias is applied entering a curve.
Different programs display different curves when bias and tension are adjusted, and Xara has no controls for applying bias or tension to a curve per se, but you manually achieve these math terms by adjusting the control point in the middle of a curve.
I?m just trying to provide terms here for effects we use every day, but might not have words for them!
My Best,
Gare
Bias and Tension 
Linear Mode
