Why captive? Just because you have to free it!
As you can notice, if you're a bit aware of this kind of material, this puzzle is made out of corian, which is not a natural material. It looks like stone.
The goal is easy, how to achieve it is a bit more complicated, especially because you cannot really see the mechanism. However, you can still have an idea of it.
What can you hear? Thing(s) ratling inside.
What can you do? You can depress the lid!
Now you understand that you need to depress it enough so that the coin reaches the same "level" as the hole to free it.
If you have ever played with this puzzle, I am sure you have managed to solve it several times, but not always on the first attempts. The hidden mechanism (simple, if I am right) is the roots of that.
A unique design, crafted by Frank Chambers, this puzzle is also pretty scarce.
The difficulty is not high but solving it is pleasant, and remember that usually with Frank Chambers' puzzles, don't expect to spend hours and hours on a single puzzle.