Let me be the first of the twenty: Lu is wrong. Run a pprune search and you will get a detailed discussion on this issue.
The Bell 212/205/204 series helicopter has it's transmission mounted by 5 transmission mounts. Like any mount, these allow a slight flexibility in movement and they provide a dampening effect on the vibrations of the rotor system so that they do not interact with the fusleage in an excited way (much the same as my wife's role preventing my excited interaction with other female bodies!). Not hard to visualise (despite Lu's comments) but the two bladed system of these aircraft has a certain vibration harmonic, just like every rotor system and other rotating part on earth. When that vibration harmonic is not effectively damped out by the transmission mount, you get "pylon rock".
The mounts can be tightened, or losened to provide the correct damping function, and can easily be tested in accordance with the maintenance manual. Simply sit at a 10 ft hover and move the cyclic rapidly fore aft until you feel a harmonic in the fore aft axis building up. At that point, freeze the cyclic in the hover position, and count how many more oscillations are felt before they stop. Typically, you should feel about 4 (from memory I think the limit is 3 to 6), and they get progressively weaker as they are damped by the mount. If you get less. the mount is too tight and will wear out quickly, if it is more, tighten the mount. If you cannot tighten the mount anymore, replace them.
Pylon rock also gets confused with SCAS induced oscillations in the 212 at low speeds, particularly in out of trim conditions. If this relates to your question, let us know.
Standby for Lu's 214 moving transmission story that has nothing to do with your question...
