A helical camshaft is a type of mechanical variable valve actuation (VVA) system. More specifically it is a camshaft

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

 that allows the valve opening duration to be varied over a wide, continuous, step-less, range, with all of the added duration being at full valve lift.

In this article a “variable duration camshaft” refers to a camshaft with a design that is intended to replace a conventional camshaft in a cylinder head and operate the valves through conventional followers.