Oki thanks a lot ! 
it's a bit more clearer.
So just to be sure I understood well.
let's say I want to read the register 0x03 of an SPI component
As soon as I set up the pin DATA to output, that put the SPI component in a kind of "listenning" mode. And each time we set the pin Data to high it write a value to set the register we want to read.
Then to read the register 0x03 we set to high the bit i=0 and i=1.
And then only after we switch the data pin to input to end the process of writing.
Am I right?