It’s kind of a religious trance. She’s experiencing things from Mercer’s POV from what I recall. It’s kind of like she’s narrating it, AND he’s narrating it. The user of the machine is almost literally put in Mercer’s shoes.

The Wheel of Time series scratched that itch for me. Evil forces have been planting seeds of destruction for generations in wait of their dark overlord’s return. There’s an organization of powerful female magic users. Male magic users go insane due to something that happened the last time Evil and Good faced off.

It also has a “game of houses” that seems like a direct inspiration to GRRM’s Game of Thrones. Lots of backstabbing and secret alliances. Trust no one kinda stuff.

Plus it’s finished, and I’m losing hope ASOIAF will ever get done.

They wanted to see how Rachel not knowing would impact the test results. If they told Deckard, that’s another fly in the ointment for the experiment. They were basically conducting a double blind test.

Edit: the owl was for him to keep his mouth shut. Rachel would have to be killed if anyone found out.