The cross-flute fold is the best way to fold twinwall if you need a very accurate and dependable change of direction.
We call it a 'cross-flute' fold as the technique can only be applied to fold lines that cross over the flute walls - either perpendicular to the flute direction or at an angle. The technique cannot be applied to folds running parallel to the flute direction.
How to make a cross-flute fold
Simply place a guide ruler on the line you wish to fold - paying special attention to which side of the sheet you should be cutting. A cross-flute fold always folds away from the side of the sheet you cut!
Lightly draw a sharp blade down the line so that you are effectively cutting about half way through the thickness of the sheet. You do not want to cut all the way through the sheet so it's best to err on the side of caution and initially 'scratch' the surface of the material.
You should now be able to snap the twinwall into a perfect fold as shown in the movie below. If you cannot break the fold then cut the line again, cutting slightly deeper until the fold can be made.