A delicate surface is a diversion under which lies a protective shield, a strength or a barrier. The oldest form of shield was a protection device used to block attacks. Many surviving examples of metal shields are generally felt to be ceremonial rather than practical.
In addition to manual dexterity, a sleight depends on the use of psychology, misdirection, and natural choreography in accomplishing a magical effect.
What appears to be one thing is often misleading, seems solid but is the most delicate surface, easy to change or destroy but the basic elements remain.