Hi!
Here's a pic. A conventional one, though.
Rather than a coil spring, they developed a way of using compressed air. On the end of the valve is the cam follower that doubles as a small piston. This runs in a cylinder mounted on the cylinder head. Air is trapped in the cylinder and as the cam opens the valve, the air is compressed. When the valve is released, the air in the cylinder acts like a spring expanding to close the valve again. In the real world, a full cylinder of air at atmospheric pressure does not have enough force to close the valve again so the cylinders are pressurised to around 100psi. Obviously no seal is perfect and the teams expect some leakage so the cars carry a reservoir of nitrogen compressed to more 2500psi. This is regulated and fed to each of the valve spring cylinders via a ring main and keeps them all topped up to the required pressure. Nitrogen is used because of its stability, however, with the changing temperatures of an engine, pressures would also vary wildly. To combat this each cylinder also has its own bleed valve to release excess pressures. This has the added advantage of bleeding off any oil that collects in the cylinders.
The principle is remarkably simple to the point where one wonders why it has not been thought of before. It produces a lighter valve assembly and does not run out of travel as easily unlike a coil spring that can become coilbound. The tricky part, though, is the design of the seal, a part that is crucial to the effective operation of the valves. Leakage must be minimal as too much gas loss will eventually bleed the system resulting
in valves hitting pistons and the kind of spectacular retirements we see on TV.
The seals must also give a degree of friction to damp the valves movement. When the cam punches the valve open, its inertia can cause it to keep going so a little friction to help slow it down is essential. This used to be achieved with double valve springs mounted one inside the other. Made to precise tolerances, they would rub together giving that crucial source of friction. It now comes from the seal rubbing on the walls of the cylinder. With this technology, Formula 1 engines now rev to some 18,000rpm with consequent increases in the all-important power.