  The quantum dynamics away from equilibrium is of fundamental interest for
interacting many-body systems. In this letter, we study tilted many-body
systems using the effective Hamiltonian derived from the microscopic
description. We first give general arguments for the density relaxation rate
satisfying $1/\tau\propto k^4$ for a large class of systems, including the
Fermi Hubbard model case as observed in the the recent experiment [1]. Here $k$
is the wave vector of the density wave. The main ingredients are the emergence
of the reflection symmetry and dipole moment conservation to the leading
non-trivial order of the large tilted strength. To support our analysis, we
then construct a solvable model with large local Hilbert space dimension by
coupling sites discribed by the Sachdev-Ye-Kitaev models, where the density
response can be computed explicitly. The the tilt strength and the temperature
dependence of the subdiffusion constant are also discussed.
