Halfway into 3rd Edition, while designing Miniatures Handbook,
we realized that free actions hid a potential smorgasbord of cool
new mechanics. We subdivided the free actions into immediate
actions (a free action you can take when it isn’t your turn), and
swift actions (a free action you can take when it’s your turn).
However, it wasn’t until Expanded Psionics Handbook’s development
that the bulb fully brightened. The concept that swift and
immediate actions could serve as one more resource available
to a player opened up new vistas of possibility, expanding options
in the game.