A bend rebend machine is a hydraulic testing machine used to test the ductility and durability of materials like steel bars by bending them to a certain angle, and then bending them back. Key features include hydraulic operation, a C-frame design, adjustable support rollers, and a set of interchangeable mandrels (forming tools) of various diameters to test different sizes of specimens. The machine is used to ensure the material can withstand bending and rebending without cracking or fracturing, a crucial quality check for construction materials.