Question

why it is easier to break an object from its centre?? of example- a karate performer breaks a plank or when a rod is pulled from both sides

Answer 1

An object like a plank of wood breaks when you can get it bend about a point beyond some critical angle.

Now, torque, which makes an object rotate, is proportional to the distance from which the rotation occurs and proportional to the force applied

So in the diagram here rotation occurs at the end where the supports are. Given the force F of the impact of the hand is equal, to maximize the rotation at the supports, the best place to hit the wood is in the middle, as that is where the distance from the points of rotation is maximized for both of them.