How does the GreenAddress refill (relock) work when its nLockTime expires? Ok, I understand that GreenAddress wallet uses a nLockTime transaction to put the funds in a 2of2 wallet, to give the user the peace of mind that, in the case their server disappears, they can still have access to the funds.

Now I'm interested to know how does the refill work exactly? I've read somewhere that if you make a transaction before the nLockTime expires, your transaction is automatically re-locking funds? How is this possible technically?

Thanks
For every bitcoin transaction you make or receive in a 2of2 wallet the platform sends you an email with an encrypted zip file containing all relevant transactions (spending all your utxo, unspent transaction output) with nlocktime set and partially signed (by GreenAddress).

When you normally spend on the platform, you will make some of these presigned transactions invalid by  virtue of spending those utxo (this is because when you spend you spend the entire amount and if there's any change left this is sent back to yourself, in our specific case to a new address) and when this happens you get a new zip with an nlocktime starting from when the zip was generated for the new transactions.