The acrylic - which is much thicker than an acrylic ruler - has several holes down the center of it. When you press on the wood handle/bar, it causes a row of ?? silicone, maybe ?? balls to press down evenly through the acrylic onto the fabric. Much more grip than the grip dots that can be placed on rulers. But when you aren't pressing on the handle, the balls are raised, so the ruler slides across fabric until you press down again.
If you watch the video, the easiest place to see these is at about 5 minutes. If you go to their web site, you can kind of see them here
http://quiltersslidelock.com/buy/