In terms of ships at sea, they exhibit stresses caused by wave action. Two of these are longitudinal stresses; sagging and hogging which caused by a ship heading though waves and troughs. Sagging is when the bow and stern are in the wave peaks and the middle is in a trough which bends the middle down, and hogging conversely, is when the bow and stern are in troughs and the middle is on a wave peak which bends the middle up. So, to tighten the torsion rods to cause an upward bend in the car middle is entirely reasonable to be caused hogging.
