Mounted models with a saving throw of no better than 5+ and a normal move rate of at least 6" are classed as fast cavalry. These nimble, lightly armoured cavalry can manoeuvre much more easily than slow, ponderous foot troops encumbered by weapons and armour.
During their move fast cavalry are allowed to turn without incurring any penalties on their move distance.
Fast cavalry are permitted to change their formation once during their movement by any number of ranks and with no penalty to their move.
The manoeuvrability of fast cavalry is so great that they can even take advantage of their ability to turn and/or change formation whilst marching - unlike other troops who are unable to march and turn or change formation at the same time.