How does pause/resume installating work exactly?

Okay here is my problem. I need to often install apps over a day, this is because I'm in the country and have only access to wifi hotspots. Often I need to pause and resume downloads from the App Store, I can't download an entire program in one sitting.

Often I lose what I have and the app needs to be re-installed from scratch (not fun if it's Final Cut X on a slow wifi connection). However, sometimes apps resume where they left off.

Does anyone know a way to make sure I don't lose what I've already downloaded?

I'm thinking that maybe at times, when I power down/shut off my MacBook Pro --- that's when the app store loses it's pause "point".

Any help appreciated.