(a) A disclaimer of an interest in property by an appointee of such interest in property as a result of an exercise of a power of appointment by a holder takes effect as of the time the instrument by which the holder exercised the power becomes irrevocable.
(b) A disclaimer of an interest in property by a permissible appointee of an interest in property or by a taker of an interest in property as a result of a holder’s failure to effectively exercise a power of appointment granted to such holder (a taker in default) takes effect as of the time the instrument creating the power of appointment becomes irrevocable.