A helical spring loaded by twisting rather than pushing or pulling: the two straight legs are rotated about the coil axis and the wire works in bending, not torsion, despite the name. It must be wound in the direction that CLOSES the coil under load, and it needs a mandrel or shaft inside it, because the coil diameter shrinks as it is wound up. Real springs differ in wire diameter, coil diameter, number of turns and leg length.