It rotates the inlet camshaft to alter the timing.
Early Alfa 8 Valve (1986 ish) TS engines had a hydraulic oil/revs two position variator that had a switch on the end that altered the ECU map between two settings as well as rotating the inlet cam. At that time Alfa didn't have enough cash to develop a 16 valve head for this engine and this was their very neat alternative. The idea was to give you an engine with good low rev torque characteristics, and then a second cam setting allowing better high rpm breathing. It was very basic compared with the Honda VVT system which was fully electronic, but in practice it worked extremely well and was pretty well 100% reliable..except where oil changes were ignored and they sometimes clogged up...owner error. Also on these engines the two identical spark plugs fired at fractionally different times.
On the Fiat based 16Valve TS engine the setup is a bit more sophisticated with more of a view to emissions limitation than out and out power.So the mapping is more adaptable and the variator is more variable, but has proved to be of indifferent quality and fails with depressing regularity. A few years back it was mooted that Alfa had hard words with the supplier of the variators to improve the quality, but I don't know if it made any difference. On the 16 Valve the twin plugs are different sizes but fire simultaneously. Variator failure is recognised by the diesel rattle at low revs and loss of bottom end go. Its not harmful to the engine, but not desirable either.
Thus a fundamentally good idea has become a pain in the arse