  Quantum systems that violate the eigenstate thermalisation hypothesis thereby
falling outside the paradigm of conventional statistical mechanics are of both
intellectual and practical interest. We show that such a breaking of ergodicity
may arise purely due to local constraints on random many-body Hamiltonians. As
an example, we study an ergodic quantum spin-1/2 model which acquires a
localised phase upon addition of East-type constraints. We establish its
phenomenology using spectral and dynamical properties obtained by exact
diagonalisation. Mapping the Hamiltonian to a disordered hopping problem on the
Fock space graph we find that potentially non-resonant bottlenecks in the
Fock-space dynamics, caused by spatially local segments of frozen spins, lie at
the root of localisation. We support this picture by introducing and solving
numerically a class of random matrix models that retain the bottlenecks.
Finally, we obtain analytical insight into the origins of localisation using
the forward-scattering approximation. A numerical treatment of the
forward-scattering approximation yields critical points which agree
quantitatively with the exact diagonalisation results.
