In essence the rebound spring assists dampers in controlling rebound (no surprise)
after a bump or undulation in the road has been absorbed (to a degree
) by main spring compression. It
also serves to provide anti-roll characteristics in that the inner wheel unloading in a turn is resisted by combined spring rate so tending to keep body in a flatter plane and consequently more rubber on the tarmac.
For this reason as well as ensuring there is no static compression on rebound spring, as mentioned above, its also important there is no static gap existing between rebound spring and bottom of stub axle, such that both rebound and anti-roll properties come into play soonest. This is why the "just able to turn rebound spring" static test is performed as an aiming point.
