Basically, the power alliance was told that they were all immune due to being in the alliance. Then last night, the nommed HMs could save one of themselves and put a power HM up. They saved Ash and put up Ashleigh.
The problem is that the twist was unplanned. They told us lies, they even told Rylan and Emma lies judging by the fact that they only just found out that the producers did this last minute by asking "why is Ashleigh up, I thought she was immune?". The Ch5 press release also stated that power alliance would be immune".
So basically, it was all a lie.