I obviously don't understand cavalry warfare, stirrups, or both very well, because I can't see how a little footrest suddenly made cavalry the shit.
The use of stirrups allowed for one to better couch his lance. Before this, lances were used more akin to spears, or with an awkward, double-handed grip, when they were couched, it posed a serious risk of falling off to a rider. The combination of stirrups, improvements to the saddle (high arch, strong back) made it more effective than before. A stirrup increased control, balance and poise of its rider - something quite useful for a chap who is swinging a sword often the length of his torso, or using a Lance as tall as he is.
Initially, in the 1960s, when this theory first emerged, far too much stock was placed in the stirrup's effect, with a man by the name of White loudly declaring that the stirrup led to the birth of feudalism. An overly-bold claim.
The stirrup allowed a charging man to use his feet to control his horse, and to put weight on them, allowing him to adopt a stance that focused all the energy of his charge into a single point - that would be, ideally, the tip of his lance. This is, again, however, an oversimplification, and the 'couched lance' did not suddenly blow away more archaic forms of warfare. 1066, and Hastings, which is often recognized as the birth of the dominance of the European 'Knight', probably didn't have any couched charges. Near contemporary pieces, such as the Bayeux tapestry, seems to confirm this. 'Shock' Cavalry was in use for a very, very long time before the introduction of the stirrup, so be careful when reading secondary histories that claim it utterly changed the face of warfare. It was only yet another aid for the development of the combat horseman.
This is about 700 years earlier than where my real expertise of Cavalry warfare begins, so I sincerely hope a medieval flair will see this thread.
hi! the threads I've seen state that importance of stirrups has been exaggerated; here's /u/Tiako on the subject:
incidentally, if you're keen on stirrup info, here are other discussions on the development / use / importance
Where were stirrups first invented and widely used? And why did it take so long? - Ancient China
Roman Legions v. Stirrup Equipped Heavy Cavalry - Ancient China
Why did it take so long to invent stirrups? - Huns, Avars, Byzantines
How important were stirrups to mounted warfare? What, other than stirrups, could have led to the rise of shock cavalry? - Ancient Greece, Ancient Rome, Franks
Why did it take so long to invent the stirrup? - Ancient Rome
Just how effective and were slings as weapons in ancient warfare and how were they used? - various
Battle of Hastings and Stirrups - Battle of Hastings
How did heavy cavalry lancers work? - Battle of Hastings
