Yeast Hybrid domain - (Apr/13/2008 )

Can some one explain me the Principle behind the Yeast hybrid mechanism for protein and protein interactions.
for example One protein X which is Ntail of receptor and i want to know which protein interacts with this N tail "Y" i have to do Yeast Hybrid assay.
So N-tail will be used as bait and i have to make a cDNA liberary of the cell where it express this recepotr and will express this as fish.
Do i am right.
Now whats next,
Why the two selection procedures.

First you have to transform your plasmids into the cells and to check if both plasmids are correctly taken up. The second selection is then the real Y2H assay where you select on positive interactions. There you can use systems which express His3 or ß-Galactosidase as reporter genes.