Thanks a lot for explaning,
Here is what i got  from that
For example i have a Variable a and i perform some operations like addition , subtration etc and then pass it through a model for backpropagation , so  those subtraction and addition are still in history of a and it will try to backpropagate to that point , so if i dont want those operation to be considered in backpropagation, i just have to detach that variable to the last consideration point
is that right ??