The Satan knows when he's getting stronger, but he doesn't know when he's getting weaker. Through hardships and suffering, we are able to take back the oros that the Satan gained from our sinning. To seduce, to prosecute, and to destroy are the three functions of the Satan. When a person sins, a klipa comes on him and he loses the oros to the Satan. The retrieval of these oros are dependent on the suffering that comes upon the sinner.