It isn't completely risk free because someone can issue another interfering order at the same time, but the basic process is this:
Check that there are no current expired orders, no bids high above the feed, and check the interest rate and price limit for any current shorts. Create a short order in the gap between highest bid and lowest ask and click short, but don't confirm it yet. Create a matching buy order to buy up all of your short, but don't submit it yet either. Double check the feed and order books to make sure your orders will currently be matched with each other, and then quickly confirm both orders so they make it into the same block. Assuming no one else submits an order in the same block that crosses over your order price, they'll match and you can use the BitAsset from your new short to cover your old short, effectively recollateralizing and extending the expiration to 30 days.