Function TBpsider transforms vector x as follows
\[
TBpsider(x)=
\left\{
\begin{array}{cc}
1- (x/c)^2 [6- 5 (x/c)^2] & \mbox{if} |x/c|<=1 \\
0 & \mbox{if} |x/c|>1 \\
\end{array}
\right.
\]

Remark: Tukey's biweight psi-function is almost linear around $u = 0$ in accordance with
Winsor's principle that all distributions are normal in the middle.

This means that $\psi (u)/u$ is approximately constant over the linear region of $\psi$,
so the points in that region tend to get equal weight.