An ASR lets a company cut its share count in one day rather than over quarters. The bank borrows shares through securities-lending to deliver them, then buys stock over the following months to return the borrow. Final settlement is based on the average price over that window, usually volume weighted.
The effect on the market is a large immediate reduction in outstanding-shares plus a persistent bid from the bank's hedging for months afterward.
Example: a $1B ASR with the stock at $50 delivers about 16M shares upfront, 80% of the expected 20M. If the vwap over the buying period is $48, the final count is 20.8M and the bank delivers 4.8M more shares at settlement.
Related: share-buyback, vwap, outstanding-shares