It's better to use the change shield button to update the direction of your swing..with the block button it gives a pause to your swings, slowing you down. The AI does the swing direction faking thing a lot, especially the champions. However you can just spam block/parry every time you hear the sword whiff (you hear it when they're about to swing), and you're pretty much invincible.
Another great thing about the switch weapon/shield button is they draw your arm back no matter what, so you can use it to reduce the recovery time of attacks. This way I can get about a 50% increase of hitting speed.. just hit switch right as the weapon makes contact.. excellent for killing someone knocked down or wiping out crowds with quick multiple blows.
It's also what I currently use to correct the wrong swings, say a side to side instead of an overhead, but still it's not always accurate and unreliable in tense situations, say when you just got knocked of your horse and are surrounded by a 5 guys, the directional arrows beging to screw up with so many targets close by =