Mandrel bender - GIF

[Saltfever]

Look at 0:43. Those cups in the vise are lubricated to help promote slipping. As Norton pointed out, sometimes, balls are used. "Mandrel" is the operative word that tells you something rigid is inserted inside to prevent collapsing. I have used sand, melted solder, melted lead, etc. After the bending the tube is heated and the low temp metal flows out. Cleaning is always a problem. I have even filled with water and then froze copper tube with mixed results. Both ends have to be crimped shut and then cut off later. I don't think those methods are considered "mandrel" bending.