How force works on two strings connected by a mass

So if I slowly pull the A string first B string will tear apart... Cause the A string will expand and it will pull the mass and B string... So B string will reach its breaking weight before A... Cause B has an additional weight mg working on it..

But if I suddenly pull the A string,it will tear apart but the B string will remain intact... I am trying to understand why this happens... I mean what is the exact mechanism here?

Attached Images

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