
Unfortunately, whether Download Booster actually works this way is something that's unclear. According to one XDA user, enabling Download Booster gets him higher download speeds when using the SpeedTest app, though others say it doesn't do anything when you're actually downloading files. A single line of code in the system files can apparently be changed to enable Download Booster natively, but at the moment that isn't possible, since there is no root exploit available for either variant (though that should change very soon.)
If you want to test Download Booster on your AT&T or Verizon Galaxy S5, download a third-party launcher like Apex or Nova, long press on the homescreen, then choose Shortcuts » Activities » Settings » Download Booster to create a homescreen shortcut. Let us know if you see any noticeable difference in download speeds after you enable it.