Here we have a 993tt lifter. Initially poorly understood and very much under appreciated(read inexpensive). Today, however, after 10 years it is very much appreciated (read expensive). This guys has donated itself to science.

Mechanics: The whole functional part of this array is found at the right hand side. That cylinder, far right, has a tiny hole on its side. This smaller cylinder enters the bigger cylinder, far left side, with such precision that it actually compresses air as it slides in. In time, however air will leak out. Oil is pumped thru the open mouth of the smaller cylinder on the right with high pressure. The oil is forced into the small hole on the side where it makes its way to the chamber it makes with the larger cylinder. The 0-ring prevents the oil from bleeding out, and so the pressure gets so high it's very difficult to compress the cylinder within each other....they come apart only as much as the valve stem allows it. This effectively takes up the slack between the valve stem and the "foot". There is a small spring to assist in maintaining a "potential space" to fill with oil. The plate at the far left is actually a swiveling plate on the side you see, and a flat surface on the other side which mates with the valve stem surface.