The Flowline technique projects the isolines from one surface onto the surfaces of the feature. The isolines are projected in the direction of the surface normal. In the example below, the isolines of a flat surface are projected onto the part. By using a side mill cutter, flowline machining can be used to machine undercut regions.
A flowline milling operation is created using the following steps:
Click the Features step.
Click Surface Milling. Click Next.
Select the part surfaces and click the plus button.
Click Choose a single operation and click Next.
Click Flowline and click Finish.
Double click on the feature to edit it.
Click on the word, flowline, in the tree view and click on the Surface Control tab.
Click on the Pick surface 
button
and select the flowline guide surface.
.
The normal of the flowline guide surface must
point toward the surfaces of the part. When
you click on the surface control tab, two arrows are displayed on the
flowline guide surface. In
the figure below, the normal correctly points
toward the part surfaces. If
the normal needs to be reversed, click the Switch machining side button
.
The arrows also indicate the starting point of
the surface isolines and their direction. In
the picture above, the toolpaths that will be projected will start in
the lower left and will follow the rows of the surface. Use
the Set isoline row/col 
button to change the starting
point of the pattern and use the Cut direction button to
change the direction of the cuts.