Flip: Curve don't move, stay in place but its "way" is inversed! Start becomes end, and end becomes start!

It's the same when you draw a curve from left to right or right to left : it's way is inversed!

Here with the function Linweb, so the result is very different!
Lineweb : http://moi3d.com/forum/index.php?webtag=MOI&msg=3666.1