  The Fredrickson-Andersen one spin facilitated model (FA-1f) belongs to the
class of Kinetically Constrained Spin Models (KCSM). It is a non attractive
process with positive spectral gap. In this paper we give a precise result on
the relaxation starting out of equilibrium for this process on an interval
$[1,L]$, and show that this process exhibits cutoff at time $L/(2v)$ with
window $O(\sqrt{L})$ for a certain positive constant $v$. The key ingredient is
the study of the evolution of the leftmost empty site in a filled infinite
half-line called the front. We show here that the front evolves at linear speed
which coincides with $v$ and further that it satisfies a central limit theorem.
