  The kagome geometry is an example of frustrated configuration in which rich
physics takes place, including the emergence of superconductivity and charge
density wave (CDW). Among the kagome metals, ScV$_6$Sn$_6$ hosts an
unconventional CDW, with its electronic order showing a different periodicity
than that of the phonon which generates it. In this material, a CDW-softened
flat phonon band has a second-order collapse at the same time that the first
order transition occurs. This phonon band originates from the out-of-plane
vibrations of the Sc and Sn atoms, and it is at the base of the
electron-phonon-coupling driven CDW phase of ScV$_6$Sn$_6$. Here, we use
uniaxial strain to tune the frequency of the flat phonon band, tracking the
strain evolution via time-resolved optical spectroscopy and first-principles
calculations. Our findings emphasize the capability to induce an enhancement of
the unconventional CDW properties in ScV$_6$Sn$_6$ kagome metal through control
of strain.
