really? how sure are you on breaking a person's arm in 2 sec?

yes, I want to learn the theory.

If a person give you a punch, grab this hand and put it on your shoulder, hold it.  And give pressure to down on the middle part.  For sure, you will break his arm.  

in theory, yes

I am not trying to hurt anyone, but just disarm the attacker's ability to attack me asap.

if someone attack me.

if you are able to grab his hand

Or just break the thumb...will help.

and given he would stand still and let you apply pressure to his joint

I don't need to grab his hand...if he punch me...I just need his punch is on my left should ( i have to escape the punch)...then i just give pressure down in the middle of his arm.  Then I can break it.