From this page: http://d-programming-language.org/expression.html

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Casting to a CastQual adds the qualifier to the type of the UnaryExpression.

Casting to the empty ( ) has the effect of removing any top level const or immutable type modifiers from the type of the UnaryExpression.
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When I read this, I interpret casting to a CastQual as only *adding* the given modifiers, but it actually *replaces* the modifiers.

For example:

shared int x;
assert(is(typeof(cast(const)x) == const int));

Note, shared is *removed* from the modifiers.

For the second statement, casting with ( ) removes shared as well:

shared int x;
assert(is(typeof(cast()x) == int));

Finally, "Casting to the empty ( )" reads poorly.

I would say "Cast with no Type or CastQual inside the parentheses has the effect of removing any top level const, immutable, or shared type modifiers from the type of the UnaryExpression."

I also would like to see examples here, feel free to use my two above.