Actually, as it have been said before, it's the master that sends the NACK.
Here is how it goes:
1- master (M) says to the slave (S) he wants to read @ a certain address
2- Slave acknowledge that
3- Slave start sending the needed data, byte by byte, and with each byte sent from the slave, the master acknowledges it.
4- This continue until the master have enough data, so it does not acknowledge data sent from slave. That means in plain English : "shut up!"
5- only then the master can regain control of the bus
any clearer?