It depends on the platter. The old VPI TNT platters we're recessed in the center with a washer around the spindle, so when the clamp was turned down the record was held flat across the entire surface. The result was the entire surface was part of the platter. Worked well if you did not "over tighten" the clamp, resulting in "dished" record.
Vacuum hold down also works in most cases.
Vacuum hold down also works in most cases.