This add on refactors the checkout process to do three things:

1. Insert orders in the database before redirecting to external payment processors (e.g. PayPal).
2. Shorten the checkout registration process by creating the account implicitly rather than explicitly.
3. Accelerate the checkout process by skipping steps where the user was just clicking a continue button.

Forums support thread: http://forums.oscommerce.com/index.php?showtopic=344987

More...