When a sheet is bent by a punch and die, part of the deformation is permanent and part of it is elastic, the same way bending a stiff wire has a part that stays bent and a part that wants to straighten back. Once the punch lifts away and the bending force disappears, the plastic part of the deformation stays locked in because the metal has permanently shifted, but the elastic part relaxes back like a compressed spring being released. That partial return toward the original shape after the tool is removed is exactly what die designers call spring back, and it is the reason bending dies are made with a slightly larger bend angle than the final part requires.