  This paper presents an interpretation based on gamma-ray burst source
kinematics for the relationship found by Norris et al. between peak luminosity
and energy-dependent pulse lag. I argue that the correlation should instead be
between "number" luminosity and pulse lag. This interpretation improves the
least-squares fit of this correlation for the known bursts by 25 percent or
more. It also suggests a distance estimation scheme. I propose that this
relationship is due to the variation in line-of-sight velocity among bursts.
This interpretation allows one to speculate on the range of gamma-ray burst
expansion velocities or the size of their jet opening angles.
