Usually something like 48 hour read-only, and restricted to some BS viewer that doesn't allow printing.

Fairly trivial to bypass usually, worst case is just printscreening it.

Edit: Ah -

    View the article PDF and any associated supplements and figures for a period of 48 hours.
    Article can be synced in the ReadCube Cloud.
    Article can be viewed offline in ReadCube's desktop and mobile apps.
    Article can not be printed.
    Article can not be viewed or saved outside of ReadCube.