OK so I'm confused about exactly what "Set off balance" means.

For example, the the dual wielding line, when you morph Flurry to "Blinding Flurry" you get a 4% chance on hit to "set target off balance". Is this just fancy talk for a knockdown or something else? I've searched for what this means exactly but no luck. If anyone could clear up this ambiguous language I'm sure it would help others as well as me.

It's more like them taking a step back and for them to regain focus again.

Imagine in a fight someone just gets a quick punch to the head and that person stumbles back then continues to fight. Just like that

I'm really looking for the utility explanation behind it. Is it an interrupt? Does it cause the mob/player to miss their next attack? Is it a half second root? I haven't seen the mechanics of "set off balance" fully explained anywhere.

while on the topic, what exactly is disorient?

i THINK (i may be wrong) is that once they are set off balance and you land a power attack in that time frame, you knock them down.

Disorient seem to be a stun that breaks on damage ( i might be wrong)

Disorient is like a mez, breaks on damage. Think set off balance is when they get all dazed and wobbly with the circle above their head and are open for a heavy attack.