It is normally a given that a key has to be a hard rigid object, in order to insert it into the lock. However there is supposed to be an exception to every rule, and the Chain Key is the exception to the idea that a key has to be a hard, rigid object.
The actual key is totally flexible, and it sits inside a metal channel. Offer the chanel up to the keyhole, and then slide the plastic handle, which causes the key to be inserted into the lock. Because the keyhole is the same width as the key, the key cannot flop about, and it slides smoothly into the keyhole.
You get two keys with the lock. The lock is not a padlock or anything like that; it is basically just the barrel of the lock. This is more a demonstration of the concept.
Tim suspects that the internal channel inside the lock is not straight, since a flexible key like this would have no problem negotiating a non-straight channel.
We only have a small number of these.