The term Iaidō (居合道) refers to a specific style of kenjutsu that is concerned with the smooth, controlled movements of drawing the sword from its scabbard, striking or cutting an opponent, removing blood from the blade, and then replacing the sword in the scabbard. The linear motion and force applied to draw the sword from the scabbard results in slashes that are faster and stronger than ones executed with the sword already drawn.
The speed at which these techniques can be executed — the initial strike in particular — is such, that when performed by a skilled user, it can prevent enemy shinobi from forming hand seals (and thus ninjutsu), by exploiting the small opening that these movements create. This makes them highly effective against most shinobi.
Once per Turn the user of this style can use a Reaction to attempt to stop an enemy from using one of the Jutsu they attempted to activate on their turn. Your Striking Speed must be higher than the target's Speed for this to work.
At 10 Points in Bukijutsu this Reaction effect can be done Twice per Turn.
In practice this means that if an opponent is already within melee range for you, you can disrupt their hand seals and prevent them from doing one or more ninjutsu if you have the actions available for it.
Keep in mind that the Jutsu they are using must require hand seals for this to work. (Ninken techniques always can be interrupted despite the use of hand seals)