It charges the phone from 0% to 50% in 30 minutes, then reverts to normal charge.
Since you should never allow the battery to drop below 40% (unless you own stock in a lithium battery company or a lithium mine), it's more of a marketing gimmick than anything else. (Also, it's a full 1C charge - it charges at 3.22A - it shortens the battery life a little. A 0.75C charge [about 2.4A for the Note 4] is a lot better for the battery.) If you want maximum battery life between new batteries, charge it on normal charge mode (not fast charg) when the battery gets down to 40%-50%. (If you need more than that between charges, buy a spare battery. (You can get 2 with a charger to charge one of them outside the phone while charging the other one inside the phone
for $23.)
If you want 6 months life, you can let the battery keep running until the phone shuts off, then use fast charge to get half a charge in half an hour.
To address your last sentence, if it's already near 50%, you get the normal charge time to 100%. It's only that first 50% that's fast (and harmful, both because of the charge rate and because you let the battery discharge too far).