swfshape->drawcurve() draws a quadratic curve (using the current line style,set by swfshape->setline()) from the current pen position to the relative position (anchorx,anchory) using relative control point (controlx,controly). That is, head towards the control point, then smoothly turn to the anchor point.
See also swfshape->drawlineto(), swfshape->drawline(), swfshape->movepento() and swfshape->movepen().