Dual air forks negative positive chamber pressure equality
The negative air chamber provides resistance to the positive air chamber. It controls the speed air spring returns to its initial shape. The chambers are isolated so the air is unable to equalise.

The positive chamber provides resistance against compression, essentially how hard is the fork.

The third part of the suspension action is dampening, the negative air chamber controls the forks return and the damper stops if from oscillating and provides a smooth ride.

Rockshox explain dual air below quite well in suspension terms.