swfshape->drawcurveto() draws a quadratic curve (using the current line style, set by swfshape->setline()) from the current pen position to (anchorx,anchory) using (controlx,controly) as a control point. 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().