Tome of Battle does a poor job of explaining the Martial Discipline Weapon enhancement:
It says that if you know a maneuver from the discipline the weapon is keyed to you get a +1 bonus to attack rolls. If you are actually using a maneuver from this discipline then the bonus goes up to +3.
Q166A: How long does this benefit last? If I decide to use Dancing Mongoose at the beginning of my turn do I get the +3 for all attacks that turn, or just the bonus ones granted by the boost? What if I use Defensive Rebuke? Do I get it on all the attacks of opportunity as well?
It goes on to say a weapon can have this enhancement multiple times as long as it's keyed to a different discipline each time (so you can have a +1 devoted spirit glaive, or a +1 shadow hand tiger claw kukri). Okay...
Then it says if you know maneuvers from both disciplines the weapon is keyed to, the bonus goes up to +3
A166a: Yeah, the description is stupidly obtuse, but at least the examples are technically accurate. Basically, if you have a weapon with discipline focus, you get a +1 bonus to hit if you know a maneuver from that school. If you are actively using a stance from that school, or a maneuver from that school, the bonus increases to +3. This means that the improved bonus applies to the attack rolls made as part of those maneuvers (or anything you do while in a stance). If you are using a boost, a counter, or a strike, you'll get the +3 bonus for any attacks that the maneuver makes you roll, but not for any others, even if the maneuver still has a duration still ongoing.
For example, you would get the +3 bonus on the opposed attack roll for Wall of Blades and the attacks granted by Dancing Mongoose. You would not get the bonus on other attacks in the round you initiated Dancing Mongoose since you only make the 2 or 4 attacks granted by the maneuver while initiating it, everything else is before or afterwards. Likewise, Defensive Rebuke grants no improved attack bonus, as you don't make any attacks as part of the maneuver, you only boost the ones that come later.
Q166B: Is this effect cumulative? If I'm a Master of Nine with a +1 desert wind stone dragon tiger claw iron heart shadow hand diamond mind greatsword, do I get a +11 on my attack roll?
Finally is says if you are actually using maneuvers from both disciplines, specifically stating a Shadow Hand strike while in a Stone Dragon stance as an example, that the bonus goes up to +7
A166b: You can have a weapon with multiple discipline focus properties. The effects of each stack. So, if you have a +1 Shadow Hand, Tiger Claw kukri, you'll have +1 to hit from the weapon's enhancement bonus (as always), +1 if you know a maneuver from Shadow Hand (or +3 if you're using one), and a separate +1 or +3 for knowing or using a Tiger Claw maneuver, for a total of anywhere from +1 to +7 to hit, based on which schools you know from and are using at any given time.
If you are a crazy Master of Nine who somehow manages to initiate or be in stances of all 9 schools at once, you could generate an attack bonus up to +27.
Q166C: Do stances count as maneuvers for this enhancement? If I'm attacking from a Devoted Spirit stance with a devoted spirit weapon do I automatically get a +3? Does this then increase to a +7 if I use a Devoted Spirit strike while in a Devoted Spirit stance?
And does this effect stack with itself beyond two disciplines? What if I'm a Ruby Knight Vindicator in a Devoted Spirit stance that uses a Shadow Hand boost, then takes an extra swift action for a White Raven boost and then uses a Stone Dragon strike? With the appropriate weapon does that get me a +15 to the attack roll?
A166c: Stances are not maneuvers. RAW, they only act like maneuvers for the purposes of meeting prerequisites of other maneuvers, qualifying for feats and prestige classes, and other situations where they are specifically called out as such. Technically, you don't get the bonus for being in a stance. However, it's heavily implied by RAI (being specifically called out as such in the example), and the line between maneuvers and stances is blurry at best. I'm not going to open that can of worms again.