as the image shows you place the tube between the 3 rollers and apply pressure to the pipe as you pull it through it will force it with constant pressure. The idea being that you set it to the natural bend of the part where it will return to its straight position. if it is bent more than that it will be bent back into shape if its straight it wont be deformed. from the test Ive done this is quite hard to get to.

If you want to see more and how I create prints and 3D designs like this check out my YouTube channel. https://www.youtube.com/channel/UCSHUc93NXGwBacoIDDqPU2w

Bill of materials

You will need 3x M8 bolts and nuts with washers.
You will need 3x of either 608z or 688x bearings
an M6 bolt around 90mm long with a nut to adjust.
For added strength you can add 2x 8mm shafts (75mm) as bearings to support the block but I haven't had to add it.