Q: keyframing in soundtrack pro

I am not sure. When I apply an effect, it will apply on the entire track, not just a region. How can I isolate a region, fade in/out the effect? Like someone walking from outside to inside a house. The ambience change completely. How can I reflect that on my track, in Soundtrack Pro...

I read about "Buses" or "Submix", very confusing info...

http://documentation.apple.com/en/soundtrackpro/usermanual/index.html#chapter=12 %26section=2%26hash=apple_ref:doc:uid:SoundtrackPro-UserManual-90762WMX-1012349

There must be something simplier than that.