Backyard engineering at its best. I'd sure like a peek at the "business" end of the rig. That looks like a Stiehl power head. He may have removed the video after realizing he'd designed a million dollar product.
It has a detector coil on the underwater end, and you search like normal. When you get a signal, you start sucking the sand up through the big black hose (like a gold dredge) that goes into the big bag. When you have sucked up the target, as it passes the coil, it makes a quick "beep," so you know you got it. Then you hear a "clunk" as it hits the other stuff inside the bag.
So, you keep on doing for this awhile, then go ashore and check to see what you got.
It looked really slick. I think it's entirely possible, technically; and it looked convincing.