Schaeffer calls the exploiting of this intellectual tension, "taking the roof off" [p. 140], by allowing the weight of these non-Christian presuppositions to come crashing down on the non-Christian. It is like preaching the law--since it exposes a non-Christian's intellectual weakness. Schaeffer cautions us not to exploit this tension any more than is necessary because by destroying a non-Christian's presuppositions, we may leave them in despair. This would be like preaching the law to someone, without preaching the gospel afterwards--leaving them under condemnation with no hope of forgiveness.
Quote taken from here.