Popcorn bidding is designed to keep a bidder from bidding at the very last second of the auction period to steal the item away from another bidder.
With popcorn bidding enabled in your Auction Settings, an item won't close for bidding until ten minutes has passed since the last bid.
For example, suppose your auction has two items, Item A and Item B, and that your auction close time of 9:00 PM is seconds away. Item A's last bid was at 8:30 PM, while a bidder just decided to bid on Item B at 8:59 PM.
When the 9:00 PM bell strikes, Item A closes for bidding, while bidding continues for Item B until at least 9:09 PM. Bidders for Item B continue exchanging bids until 10 minutes passes from the last bid.
If Item B is highly sought-after, popcorn bidding could extend bidding for hours and raise you thousands more in bids!