Depends on the crease.
If the crease occurs due to a break in the latex or primary/secondary delamination, block flat and rub some latex in while under tension.
If no latex fault, you can temporarily soften latex with a solvent (sewing softener) and put some tension on it until the solvent has flashed off. Sometimes just giving the back a hot massage with a rotary extactor will do the trick.
If the rug has been exposed to a lot of traffic while in this condition, you'll probably never get it to disappear. Sometimes the primary has stretched so much you'll tear the carpet trying to block it out, and difference in abrasion from people skidding their feet over the high spots will make for a permanent "dirt" line.