I am having some timing violations in my design with a slack of around -2100ps. I have observed that all these violations belong to a same block. So,I have traced the clock(cost_group) of that block and I have used the following "set_attribute initial_target -100 [find / -cost_group cost_group_name]". Now the no of violations in my design decreased and the peak slack is only -1600ps. Can any one please tell me what is the reason for this and what is the use of the initial_target attribute of a cost_group?