Internal kinematics directly measures a fundamental property of galaxies: mass. It can therefore be a powerful probe of the nature of intermediate redshift galaxies. The idea is simple: if intermediate redshift galaxies are as massive as ``normal'' spirals such as the Milky Way (instead of being dwarf galaxies), we would observe rotation velocities of 
200 km/s. On the other hand, if they are really lower mass objects that have been boosted in luminosity by 10
as suggested in the luminosity-dependent luminosity evolution scenario, then the TF relation predicts that their rotation velocities will be 100 km/s. This approach is direct. It is not affected by uncertainties in models based on local luminosity functions. Also, whereas luminosity functions derived from redshift surveys show the evolution of a population as a whole, the present approach can measure luminosity boosting in individual galaxies and can thus tie luminosity boosting directly to other galaxy properties on a galaxy by galaxy basis.
Luminosity evolution will shift the Tully-Fisher relation to brighter magnitudes at intermediate redshifts. If merging has been a significant process in the evolution of galaxies over the last 5
6 Gyrs, galaxies should be less massive at z 0.4. This would shift the Tully-Fisher relation to lower rotation velocities. It would seem at first glance that the two effects cannot be disentangled. However, significant mergers would certainly modify the structure (e.g. disk scale length) of galaxies as a function of time. Therefore, pure luminosity changes can be isolated by comparing the luminosity of intermediate redshift galaxies with the luminosity of local galaxies with the same size and rotation velocity.