Most people don't use a leash anymore because of natural selection
Recent kites have a good lift-to-drag ratio and people body drag back to their board when they drop it.
Everyone seems to have a story about almost getting killed due to a board leash.
On this particular day the wind was fluky and I wanted a leash. Here it is, thanks to a lobsterman whose potwarp washed ashore.
The knot is NOT A SLIPKNOT.
It's a sheet bend, figure 8, or other boyscout knot. Slide it up your leg til it stays. If your board gets eaten by a monster or dives into the sand like an anchor maybe you can wiggle out of the loop.
Still not safe, but more convenient than struggling back to the board in wrong conditions.