Lock step protection means that, before a customer proceeds to the next step, a unique code is generated, recording the current step. This may sound complicated, but it happens transparently.
There are only three steps involved, customer selects file, follows paypal button and then returns from paypal to download the file. It is that simple.
Step One
Your customer clicks a download link, reference code is generated.
Step Two
Customer given that reference code, clicks paypal button.
Step Three
Customer returns from paypal, downloads file.
During these 3 stages, there is always a reference code available. This reference code can be matched against a paypal transaction code, allowing you to grant them direct access in the event that something should go wrong. Furthermore, it enables them to have multiple download attempts, this is important for larger files or for situations where internet service had been interrupted during the download.
While this system isn't completely fail-safe (it is still possible to have a network error with paypal) you may manually set the reference code to "approved" for those cases. This enables you to recover such events as painlessly as possible.