  I show how an empirical variability - luminosity relationship for prompt
gamma-ray bursts, first proposed by Fenimore and Ramirez-Ruiz, can be
understood as a special-relativistic beaming effect in the ``cannonball model''
of Dar and De R\'ujula. In this scenario the variability is a measure of the
direction of propagation and the Lorentz factor of the cannonball on which in
turn the apparent luminosity of the prompt GRB depends sensitively. The
observed absence of cosmological time dilation in the ``aligned peak test'' -
when using redshifts derived with this relation - is also explained. The most
direct evidence in favour of the cannonball model presented here is its correct
description for the observed relation between narrow-spike width and amplitude
within a given GRB. There seems to be an indication for cosmological time
dilation in the total duration of GRBs, as expected in the cannonball model.
Quantitative predictions for the luminosity function of GRBs and the
``spectral-lag luminosity relation'' are given.
