the price changes immediately when buybacks are announced, not when the buybacks are executed. Case in point, intel jumped 4% today immediately after announcing a stock purchase of $10 billion (to be executed by end of the year), which is about 5% of its marketcap. No shares needed to change hands, the bid / ask moved instantaneously. The quoted price of a stock reflects people's willingness to buy and sell at certain prices (bid / ask), its not simply the last traded price, which is almost useless, given new information.