"The key to Hunnic success seems to lie in one particular detail whose significance has not been fully recognized. Both the Huns and the Scythians used the composite bow, then, but whereas Scythian bows measured about 80 centimetres in length, the few Hunnic bows found in graves are much larger, measuring between 130 and 160 centimetres….
"[T]he maximum size of bow that a cavalryman can comfortably use is only about 100 centimetres. The bow was held out, upright, directly in front of the rider, so that a longer bow would bang into the horse’s neck or get caught up in the reins. But – and here is the answer to our question – Hunnic bows were asymmetric. The half below the handle was shorter than the half above, and it is this that allowed the longer bow to be used from horseback. It involved a trade-off, of course. The longer bow was clumsier and its asymmetry called for adjustments in aim on the part of the archer. But the Huns’ asymmetric 130-centimetre bow generated considerably more hitting power than the Scythians’ symmetrical 80-centimetre counterpart: unlike the Scythians’, it could penetrate Sarmatian armour while keeping the archer at a safe distance and not impeding his horsemanship….
"The Huns didn’t have stirrups, but used heavy wooden saddles which allowed the rider to grip with the leg muscles and thus create a firm firing platform. Nonetheless, Hunnic horse archers would probably have been effective against unarmoured opponents such as the Goths from distances of 150 to 200 metres, and against protected Alans from 75 to 100 metres. These distances were more than enough to give the Huns a huge tactical advantage, which, as Roman sources report, they exploited to the full."
--Peter Heather: The Fall of the Roman Empire : A New History of Rome and the Barbarians