Why does it say "va'Yshabrehu" (he broke it, masculine)? Motah is feminine!
Radak: This refers to the yoke. He uprooted the stick on Yirmeyah's neck and broke the yoke tied to the stick.
Metzudas David: He broke the Kesher (knot) on the stick. Kesher is masculine.
Why did Chananyah break the stick or yoke?
Malbim: Whenever a Navi does an action with a Nevu'ah, even a Nevu'ah for evil is always fulfilled. Yirmeyah did an action with the Moseros and Motos, so his Nevu'ah had to be fulfilled. Chananyah broke the stick, to nullify the action via another action 1 .
Perhaps he did an action to accompany his Nevu'ah of breaking the yoke of Bavel, to show that it absolutely guaranteed to be fulfilled, even though in any case a Nevu'ah for good is always fulfilled. (PF)
