I was watching a random YouTube review on a Dewalt jigsaw and the reviewer did some demo cuts on some material that was clamped down with this contraption. It looks like a specially angled metal bar with a non-marring pad on the tip. He just whacks it down once into the hole and it locks secure. The a quick lateral wack on back (left to right in this pic) and it's instantly free. I was impressed!

Anyone know what this is called?

Holdfast clamps - Example.

There are a lot of accessories for those holes - look for "bench dog" clamps

Thanks for the quick answer! I just can't believe I've never seen these. I've watched a lot of woodworking videos over the years.

I think I'd have to double my benchtop thick for these to work, but I'm interested in trying these some day.

One note is that holdfasts are the older tool and here you will find them for 3/4 holes. A lot of modern dog accessories are European built and for 20mm. Holdfasts do need thicker table, the more recent stuff (from say festool) doesn't need as much thickness.