Maybe they did it after everything was covered. The big thing on the bottom looked like a winch and you could see pulleys on the frame. So I believe it's a pulley system linked to the winch at the bottom of the pool. Not a hydraulic system. I wonder what they will do with the hole on the left and right though. I also was wondering what they were drilling at the top in the beginning. But since they attached the frame to it I can only assume those are the safety pins which lock and retract when floor reaches the top or wants to go down respectively.