The desmodromic valve system is a type of valve actuation mechanism that uses gears and cams to open and close the valves rather than conventional springs. In a conventional valve mechanism, the valves are opened by a cam and closed by return spring, whereas in the desmodromic valve system, there are two cams and two actuator gears, each for positive opening and closing without using a return spring.
This system was invented at a time when spring failure was common in engines that run at high speeds. The springs could handle valve closure at low RPMs, but as the revs increase, the springs have to handle very high pressures, and following the precise timing of valve closure becomes difficult. This leads to a drop in performance, and increases chances of valve failure. The desmodromic system eliminates the spring from the system, actuating valve closure mechanically, using cams and gears. This offers more consistent valve timing and reliable performance.
Spring technology has advanced ever since, but this unique system is still used by Ducati on both its road and race bikes.
Desmodromic Valve System on the 1199 Panigale's Superquadro engine: