Never heard of that provision either. But what I do know is that the will can be "Varied", in other words the details of the will can be changed to give different benefits to different people - with mutual agreement. Strange... but true.
I think this is because the UK charge a tax on the estate of the deceased so varying a will makes no difference to them, whereas here in Ireland we tax the beneficiary, so varying a will here is a no no, as it might be done for tax reasons.
Just my thoughts, and I stand to be corrected.