I think high-speed close-up photography during a shimmy would show that the top tube and down tube have virtually no flex compared to that at the headset which is purposely intended to be a hinge. The shimmy is mainly that the headset is rotating back and forth at about 5Hz. Pressing your finger down on a string produces a discontinuity in the characteristic impedance of the transmission medium, causing a reflection of the wave. The legs against the tube OTOH reflect virtually nothing. They are inelastic, giving a very high return loss, acting more as an attenuator, absorbing the energy like the deep wedges or pyramids made of open-cell foam, fiberglass, etc. for the inside surfaces of an anechoic chamber by Soundcoat, Azonic, Foam Factory, Auralex, and other companies you're probably familiar with. The energy is turned into heat.