As mentioned on the TWC forums, it was the four-horned saddle that allowed for the couching technique, not the stirrups. This was because stirrups only offer support for sideways movement, not front-to-back. Apparently you could couch a lance without stirrups (not that stirrups didn't help give a more stable seat, I'm sure).

http://www.twcenter.net/forums/showthread.php?t=65165

Foot
Thanks for the link Foot. The discussion within is very interesting, but leaves me scratching my head. From what I've read, I gather that an actual cavalry "charge" consisted of a rapid close with enemy troops and then really more of a needling action with one's spear rather than the Earth-shaking, formation cracking, guts & glory action that we all think of when recalling mounted combat. Is this correct, or have I misinterpreted the information? If so, shouldn't the charge bonus present in EB be drastically lowered?