How does the GreenAddress refill (relock) work when its nLockTime expires?
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.