How does battery saver functionality work?

Well I’m not sure about android (if you are using an android operating smart phone). I use the iPhone myself and it has the low battery mode.

What it does essentially is it only uses data when you are operating certain apps, the screen lock happens more quickly in order to save power. Mail fetch, background app refresh, automatic downloads and some visual effects are reduced or turned off.

I’m assuming other phones such as Samsung, LG, HTC etc is quite similar.

I think it varies by phone, but for my LG Nexus 5:
“When battery saver is on…Can’t use Google Maps Navigation
Limits vibration, location services, and most background data
May not update email, messaging, and other apps that sync, until you open them”

Voice calls and texting work as normal.

It simply turns off extras that are not actively needed like gps, bright screen and other processes that draw power.