Did you read the app description? If you turn off WiFI and enable Power Saving mode, it block WIFI and Mobile DATA then the app cannot put your device in deepsleep.
During deep sleep mode, Wifi, 3G are turned off, background apps are stopped. Therefore, most apps are not working, including Facebook, Google Service, etc,. It then constantly wakes up in order to download email, sync Facebook status, etc,.
I can't really help you for the batt drain unless you could root your device to get more information from CPU: twist Process setting or buy another app on the playstore to monitor your battery drain like BetterBatteryStats.