Between the end of the job and the balance payment, many creators still have to show the client a preview for approval. The risk: that this preview gets screenshotted, downloaded or reused before it's even paid for.
The watermark, a visible but imperfect protection
A watermark on previews discourages commercial use of a screenshot, but protects nothing technically: anyone can still take a screen capture. Its role is deterrent and legal, not a real lock.
The controlled preview, a technical protection
More effective: give access to a low-resolution selection gallery, viewable but not downloadable at final quality. The client sees everything they need to approve, without ever holding the usable file.
Combine the two depending on the stakes
For a standard selection, a light web preview is usually enough. For a project with high commercial value (brand campaign, embargoed content), add a watermark on top, and reserve the final file for the post-payment unlock.
That's exactly the logic of Paylock on xFer: the client views their gallery or teaser normally, but downloading the final high-resolution file stays locked until the balance is paid.
In short
The watermark deters, the controlled preview technically prevents. Used together on high-stakes projects, they let you show your work without ever handing it over before you're paid.