In this case it is unlikely to save IHT, I meant generally. Best used where QSR is not available (say for instance because the estate of the second person to die is fully or partly exempt).
Say for example that Nellie and Gertrude are sisters and are both worth £500k. Nellie dies first and leaves it all to Gertrude. Then Gertrude dies 18 months later leaving it all to charity X. On Nellie's death there will be £70,000 IHT to pay so Charity X will inherit £930k.
However, if when Gertrude dies, the Will of Nellie is varied to "leapfrog" Gertrude and leave it all to Charity X then HMRCS have to refund the £70k since legacies to charity are exempt.
It's also useful when full use of the nrb's has not been made.