determines how the rig will behave when rotated
A rig that is in dynamic balance will pap flat. The central post will remain vertical, regardless of the speed of rotation.
What's the big deal about dynamic balance? A rig out of dynamic balance will wobble, drift, or precess around the vertical axis as it is panned.
Imagine filming an actor. Headroom is carefully set, but the sled is not in dynamic balance. At some point, the actor moves, and a pan is required. Instead of the sled's balance maintaining headroom and helping you get the shot, the rig tilts up, down, or rolls off level.
To counteract the rig's unwanted behavior, you ...