From the Grivel North America website (defunct) :
The symmetrical hanger and grip of the Helix allows the user to put in-line force on the tube to make the teeth bite into the ice easily on the first turn. Helix is built on the same polished tube as 360. The tube’s reverse profile threads eject ice outward and spread force over a greater surface area, these are hydrodynamic (radiused) to reduce friction. Helix racks conveniently (we’ve seen as many as six on a single ‘biner). The hanger does not articulate as much as 360 so it cannot be placed in the same odd locations but the Helix hanger does lift away from bulges so little surface ice must be cleared to facilitate placement. Helix offers a fantastic quality/cost ratio.
When these first came out, they were available in lengths of 15 and 20cm, and the 15cm handles were yellow.
Current models are available in lengths of 12, 16, and 20cm, with color-coded handles. See the Grivel Italy
Image from Grivel website