Seen these? They are common in the Med where they moor stern to in deep water, and that means a lot of scope with the risk of anchoring on top of someone who came in earlier with a different cross wind. It's obvious how they work to free a foul anchor. It happens so often that it doesn't even cause anchor rage, you just have to suck it up. I have seen them made of 3/8 stainless rod with eyes welded on the shank and bight and they work well too.

EC31 said..
Is there a special technique to get it to bite into the sand, or is it just designed for hooking on rocks?

Not sure I would want to trust it on a lee shore with a Southerly approaching.

It's just to untangle your anchor rode - get as much anchor in as possible then use the hook to grab either your anchor or the offending chain, then you can release the hook by letting go of the top rope while the bottom rope is secured.