With a traditional auction, you exchange contracts when the hammer goes down. This can be in person or online, but once the auction is complete and you have the winning bid, you will usually pay your deposit and have around 28 days to complete. If you do not complete within the required timeframe, there may be financial penalties.
With a modern day auction, the process is more like an online timed sale. The seller lists the property and legal pack for a limited period, with bidding open until the reserve is met. When the winning bid is accepted there may be a deposit or reservation fee to secure the deal. The process should be faster than standard conveyancing because so much of the legal work has already been prepared in advance.