Here are two more, but they're $2.99 each, not free.
The Rebellious Slave - Short story. The other stories in the anthology are pretty good too
I believe the plug part that is up inside the ass expands - small when closed for insertion, wider even opened up/expanded, making it difficult or impossible to remove without unlocking/collapsing (depending on how wide it gets vs how much you can stretch your hole, naturally).
If it's that one, there's a threaded post inside, turning it pulls another piece down, which pushes the "petals" out, expanding the plug. Then, the lock prevents it from being turned, keeping it in the expanded position.
It's based on a torture device known as the "pear of anguish" (which may or may not ever have actually been used on anyone)
Don't let the glass fool you, the round base is a bit bigger than the part just inside. It's this one: http://www.amazon.com/Don-Wand-Glass-Pleasure-Clear/dp/B0019LWKVQ/
I have no wish to lose something inside.