Hyper Mega Evolutions work similar to Mega Rayquaza/Zygaurde but with changes.
What they do:
Hypers Megas gain 150 Stat points.
You can have 2 on a team.
They only last for 5 turns.
You can Re-Hyper mega after 2 turns. You can't hyper mega 2 hyper megas on the same turn.(btw this is for double battles this doesn't affect single battles or your opponent)
They don't need items instead like Mega Rayquaza you need a Move (In Hyper Mega Mimmicrage its Spectral Grab which is basically a ghost type knock off with +2 priority, 15pp, 85 base power and 85% accuracy)
Hyper Megas CAN NOT SWITCH OUT AT ALL