  We investigate the transient coherent transmission of light through an
optically thick cold stron-tium gas. We observe a coherent superflash just
after an abrupt probe extinction, with peak intensity more than three times the
incident one. We show that this coherent superflash is a direct signature of
the cooperative forward emission of the atoms. By engineering fast transient
phenomena on the incident field, we give a clear and simple picture of the
physical mechanisms at play.
