The Mystery Box twist was featured on Big Brother 2. On day 1, Julie Chen revealed that a twist involving the surviving nominee of the first week would begin week 2. It wasn't until after the first week's eviction that Julie explained the twist to the house.
Each week four mystery boxes were released into the house. Although three were empty, one contained a secret Power of Veto. The outgoing Head of Household, the surviving nominee, and two other Houseguests - both chosen by the outgoing HOH and the surviving nominee, respectively - are able to open one each. As a result, one of the four Houseguests receives an extra Power of Veto for the week, completely unrelated to the one that could be won in the week's Power of Veto competition.
Mystery Box Selection and Usage