This flag effects the * and / operators of complex.
When defined, the "normal" formulas for multiplication and division are used. They may execute faster on some machines. However, infinities will not be properly calculated, and there is more roundoff error potential.
If the flag is undefined (default), then more complicated algorithms (from the C standard) are used which have better overflow and underflow characteristics and properly propagate infinity. Flipping this switch requires recompilation of the C++ library.