I've not disassembled mine, so I'm not 100% sure, but the ones for my bike were similar (though obviously we don't have to unlock the doors
). Basically, the keys split in two. Inside of the one with the remote controls will be a battery and a transponder (which was passive on the bike, I'm sure it is on the Leon too). Inside the other will just be a transponder. It is this transponder that the immobiliser recognises and okays the engine to start.
We found on the bikes that if one of your keys break (ie snap/bend) and you don't want to pay for a full key, you can get a simple blank cut to match and just hold the transponder-key (the original) close to the ignition barrel and it will start. If the new key has space for a transponder, you just take it out of the original and put it in the new one.
In the case of these keys, assuming they work the same (and I can't guarantee it) you will be able to open both and swap the transponders so that the spare key is no use beyond opening the doors but the main remote-key would start the car again.
As I say, I can't be 100% sure, but I can't see any reason why it would be any different. It's a fairly standard system.
Does that all make sense? I'm tired and rambling