Hard whirling machine series HWM
http://www.reitec-hagen.de/en/machines/whirling-machines/har...

What is Thread Whirling?

Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.

What is Thread Whirling?

Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.

What is Thread Whirling?

Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.

What is Thread Whirling?

Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.

Advantages of Thread Whirling-
Thread whirling offers several advantages over single point threading:
http://www.genswiss.com/whirldata.htm

Sounds obvious but I wouldn't have thought of this literal translation before I identified some native English references, such as

http://www.leistritzcorp.com/machine_tools_applications_tech...

http://www.thefreelibrary.com/Turning to hard-part turning.-... (also mentioning "hard turning")
"In the new whirling process, the steel bar stock was hardened before whirling to produce the finished lead screw. Hard whirling replaced both the soft machining and finish grinding steps. Elimination of these operations, plus the sharply increased material removal rates versus grinding, dramatically shortened the thread whirling process time to only 105 minutes."

http://www.ctemag.com/pdf/2005/0504-Whirling.pdf (p. 3)
"Chris Walther, manufacturing engineer at Danaher Motion, Saginaw, Mich., which manufactures Thomson-brand ballscrews, said his shop threadwhirls parts of different configurations and materials, and varies speeds and feeds accordingly. He estimated that 60 to 70 percent of the screws made at the facility are whirled hard with PCBN inserts, with the rest hardened after whirling. Expressing the cutting speeds in rpm, he said much of the hard whirling takes place at the thread-whirling machine’s 2,100-rpm maximum speed, while the unhardened pieces are machined in the neighborhood of 600 to 1,000 rpm."

Other contributors here have done some good research and have explained it quite well. I do see that Hard Turning and Whirling have been used synonymously. In the case of the former, obviously to distinguish between normal turning and this particular type. In the case of Whirling itself there has been no particular version of "hard" that appears to be necessary. Probably as there there has been just one 'type' of whirling in question w.r.t. machining.