The clamping elements allow lift, PERIOD . If you haven't seen lift when using them you simply haven't used them in enough scenarios. It is one of their major drawbacks. It's a simple fact, using the bottom knobs helps, but there will be lift in some set ups, with some tools. The only way to get away from lift for certain thickness and set ups is use additional clamps with down pressure.
I basically use them as side stops and they work great for that. I own 30 units, but haven't used them as much as I have in the past partly becasue of the lifting . When I first bought them I thought WOW this is the answer to my problem, well not so much. They work okay, but the design could be better.
If they offered a version that had a small lip that covered the edge of the work piece they would pull down on the work. It wouldn't allow for flush work where that lip was, but I would rather have that and just rotate the work or clamp. Or make them double sided, one side with a lip and one side one without.