You can't get the bonus from a defending weapon - unless it's changed in 3.5 - without expending an attack. You don't need to actually swing with it, but you must incur TWF penalties if you're holding it in your off hand, and if you're flurrying, one of those attacks must be allocated to the sai.
None of that is mentioned in the description of the enchantment, just that you have to choose where the bonus is allocated before attacking.
As far as disarming goes, I really don't like the idea that unarmed strikes, as light weapons, take a -4 disarm penalty.
Lotus said:On the other hand, Spring Attacking away with someone's weapon in hand does sound like fun![]()