When a strong acid is titrated with a strong base using pheolphthalein as an indicator, the color changes abruptly at the endpoint of the titration and can be switched back and forth by the addition of only one drop of acid or base. The reason for the abruptness of this color change is that:

Keto-enol tautomerism

Explanation:

Phenolphthalein is an indicator (C₂₀H₁₄O₄), which is utilized to find the volume in the titration of acid and base, Depending on the pH value the indicator, changes its color from colorless in acids to pink in basic medium.

It is a weak acid, losses a (H⁺) proton in solution, that is colorless, whereas the corresponding anion is pink in color.

This change in color of the molecule  is not only defined by the addition of  a base or an acid, but also with the structure of the indicator, which leads to tautomerism called as  keto-enol tautomerism. If a molecule has 2 (-OH) hydroxyl groups as well as 1 (C=O) carbonyl group, exhibit an equilibrium among both the groups. Because of this change, structure of the indicator changes and so color also changes.