What you are referring to is probably this.
http://www.hmrc.gov.u...ransfer-threshold.htm
Until October 2007, couples who had joints tenancy of their property found that the property automatically past to the surviving partner on the first death. When the second death occurred, the estate of the second to die was often forced into paying significant IHT - because increases in property values into recent years pushed the estate value above the threshold for IHT. There were ways around it - part of which involved planning one's wills, as well as having the property as 'tenants in common'.
At a stroke overnight, the Chancellor changed the rules such that when the second death occurred (if after some random date in October 2007), any unused nil rate band from the estate of the first death could be used by the estate of the second to die. This effectively provided a doubling to £650k in the IHT threshold for many estates - taking the estate out of IHT liability.