Known and Unknown portions of the contest are precision aerobatic sequences judged using the current IMAC rules. The Invitational class had several Knowns given to them before the contest. The Unknowns are handed out the day before they are flown.
The Freestyle is actually a separate contest within the main contest. It is a 4-minute anything goes flight where each pilot does his best to wow the judges. Having said that, there ARE criteria for judging.
They are here:
http://www.desertaircraft.com/shooto...our-Minute.pdf
As Jason points out, how low you can go is NOT one of them. Having judged more than my fair share of these things I can tell you that simply going low is not enough. The guy who wins is the one who best fulfilled the criteria.