What happens if a bid is placed within those five minutes?

Any “last second bid” placed will extended the bidding window by adding the posted amount of minutes(usually 5) to the closing time.

Example: The auctions extend time is 5 minutes and a lot is slated to close at 6:00 pm. If a bidder places a bid at 5:57 pm, the new end time will be 6:05 pm.