When sending either a read or write request to the slave, the slave is simultaneously sending data back to the master. If that request is asking for data in a register, does the slave reply immediately, or is another command required? For the RFM69, the difference between write and read is the MSB of the address. If it's a 1, it's a write request. If it's a 0, it's a read. I'm thinking, since it's an MSB first transaction, the slave knows it needs to respond and does so with the data stored at the requested address, so the slave provides the requested information during the initial transaction, but I'm not sure.