The traditional method to close the valves is by a metal spring, usually spiral wound. At higher RPM's, the springs thus have to be stronger, to force the valves back into the pockets quicker. Or else, valve float could occur, bad mojo.
Desmodromic valve closure is using the reverse of what is done to open the valves, to have a camshaft, by mechanical action, force the valves closed at the appropriate time. Thus, it opens and closes the valves machanically. This is great theory, and it has been made to work. But it requires great mechanical precision, and a lot more added mass and mechanical parts.
Pneumatic valves use a form of metal bellows to force the valves closed. The great advantage is that it has a very low amount of mass, which is beneficial to high RPM's. One drawback is that it relies on gas pressure, and if there is a leak, then the valves system can fail, bringing the engine with it.
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http://www.ducatitech.com/info/desmo.html
http://everything2.com/index.pl?node_id=1222194
