tyro,
Having "one on the shell lifter" with a Benelli means that the bolt is closed on an empty chamber with a shell on the shell lifter. There is no slide with a Benelli M1S90, as it is a semi-auto shotgun.
Here's an informative article (however, I disagree with some of its points on the relative merits of semi-autos and pumps) which you may want to peruse:
http://www.angelfire.com/tx/ShotGun/TTS.html
It, by the way, mentions that you should download by -1 to avoid spring set.
HTH,
Justin