Dry stone walls need to be filled and packed and pinned with sharp broken stones called hearting. It is like the cement of a mortared wall. The stone we work with a lot is a quarried dolomite limestone that comes in random shapes and sizes. Often we don't have enough small hearting pieces to complete a wall. Now and then we also need to shape the builder stones themselves as they might not fit quite right otherwise or they might not have a good enough face.
It always amazes me that when you shape such a stone with a hammer to get a flat face on it you often end up with the very hearting pieces you need to fit and nestle that stone into your wall. It kind of seems like the universe is trying to be economical or maybe it's natures way of telling you you're doing things right. ( Like clearing a field and making a wall with the stones instead of burying them.)
The stone that you see laying in the wall at the end of the clip, with the bucket leaning on it, has been bedded and pinned with the scalloped pieces that came off it when the stone was being shaped earlier.