What is "freestyle" footbag? Isn't that just "hack"?

Yes, basically, "freestyle footbag" is just another name for the sport that has evolved from "hacking" in a circle. The game of kicking a footbag around a circle is still the single most dominant form of the game in the world.

In freestyle competitions, players choreograph routines to music, alone or in pairs, executing difficult moves in synch with the music--the result is something like a cross between rhythmic gymnastics and figure skating.

The distinction between "freestyle" and "hacking" is in the eye of the beholder. Almost all advanced freestylers practice and play in a circle, with the same rules everyone is familiar with: (a) courtesy tosses to start the rally, (b) no hands, (c) trying to make sure the footbag gets passed to every player in the circle. (Advanced players tend to always pass the footbag in a specific direction to players around the circle so that they don't skip anyone, although even in circles of top players, this is not always possible.)

All traditional footbag "hack" circles are freestyle circles, as players not only kick the bag, but attempt to do "tricks" or FREESTYLE MOVES with the footbag while they have it.

As you might expect, there is not just one school of thought about footbag freestyle. For an alternative perspective, see "Bloughchi^tm -- A Renaissance of Modern Movement" by Steve Blough.